Because Thanos is simply stronger than the Hulk.

That statement is not made without controversy, but according to the film's writers in a MovieWeb article he didn't even need the stones,

Avengers: Infinity War has brought up more than a few debates, but screenwriters Christopher Markus and Stephen McFeely have now revealed that Thanos would've beat the Hulk even if he didn't have the Infinity Gauntlet with the Power Stone, fueling more debate. The opening scene takes place on the Asgardian refugee ship after Thanos and his Black Order have taken it over to locate the Tesseract. In a last ditch effort to defeat Thanos, Loki sends the Hulk into a rage to fight. However, the fight doesn't go in the Hulk's favor.

Stephen McFeely believes that the Mad Titan would've destroyed the Hulk with his own bare hands in Infinity War. This would explain why Ebony Maw allows the fight to continue and makes the scene look like Thanos was just having some fun, like a cat playing with a mouse. Looking back at the scene, Maw and Thanos are pretty calm, even when it appears that the Hulk has the upper hand in the battle.

McFeely had this to say when asked if Thanos could still beat Hulk without help. I think he could. I may be speaking out of turn, but I think Thanos could kick the Hulk's ass without the (Power Stone).

This was to establish that Thanos is not to be trifled with,

However, the opening Infinity War scene wasn't just to show that Thanos could beat the Hulk. Marvel Studios boss Kevin Feige revealed early on that the first 5 minutes of Infinity War would show off the immense power of the Mad Titan and he was not kidding around. McFeely explains.

From an in-universe perspective we also have Korath the Pursuer state in Guardians of the Galaxy that

Thanos is the most powerful being in the universe.

This is when Ronan is about to wield the power stone and Korath is trying to stop him.

You can also find the source interview for the above linked article here.

They appear very evenly matched in terms of raw strength, unfortunately for Hulk, strength wasn't the deciding factor in this fight.

Hulk lays out Thanos initially, and Thanos isn't able to push him off - until he's got something to brace against. In a contest of strength between two opponents of approximately equal strength, the winner is going to be the one with greater momentum or something to brace against.

Thanos uses this momentary advantage to get to his feet and set his stance, after which he controls the fight. The Hulk becomes a glorified punching bag because he fights without technique, and against an opponent like Thanos, that's not going to work.

Hulk consistently wins because he's so ridiculously stronger than most of his opponents that his capacity to absorb damage means he can basically ignore technique with the same freedom that he ignores damage.

Against a disciplined technical fighter in his own weight class? He never stood a chance.

TLDR: We don't know and can only speculate. The movie uses this to get our curiosity and to establish Thanos as a strong villain.

We never see the Hulk build up anger. For all we know he could have just woken up in his bed, realising the ship is under attack, smashing a few of Thanos underlings and then go right at him, just because he was awoken in the middle of a good dream. At the least, we know he typically gets angrier if he's in life-threatening situations himself or if the love of his life is in danger. In the MCU, Black Widow gets closest to that and she's not around, nor do we have an indication that he was in serious danger before being beaten by Thanos.

So in-universe, it is likely he wasn't at his full potential and not very focused. And Thanos is likely at least about the same strength level, with better technique (less emotional fighting style) in that fight. Hulk was probably taken by surprise in how strong his opponent was - and we've seen in Age of Ultron, that surprising him when he's not looking/focused/at full strength is a sneaky way to get him out of the picture for a while.

Hulk wasn't ultimately defeated in the sense of about to die, yet a space ship is also a terrible place for the MCU Hulk to fight in. So far, we have no indication to think that he would survive in space. Even if that were the case, breaking the ship apart would kill the remaining Asgardians. And this would likely happen if he were to fully "hulk out" and have an extensive fight against someone at least equally strong. So, in his "mentally improved" state where he can talk and recognizes "friends" properly, he might also have held back on purpose. In any case it was probably a sane decision to just send him away and not drag out the lost fight.

Out of universe, the movie explicitly doesn't focus on him or his mental state. What mood he is in, what fights he had earlier whether he just transformed, whether he already is in disagreement with Banner or not. It doesn't make a big fuss about the fight and none about any "preparation" on Hulk's side. This way it builds the question of how he lost and why he hides inside Banner up as a mystery box for the next movie to open and investigate, while for the time being using him and Thor to establish how powerful Thanos is. Up to this point, those two were the top-contenders for "strongest avenger", so it sends a powerful message if Thanos outright beats both(!) - individually.

Personal side note: I sure hope Hulk can pick up that Gimli-Legolas relationship again, solve his mental issues and "level up" as well, after Thor stepped up his game in Ragnarok and IW.

First of all this is not the Hulk of the Comics and so one can't be compared to another.

Within the MCU we have already seen Hulk beaten or matched in combat several times.

Thor is his match in the Avengers and in Ragnarok again proves to be able to match a Hulk who has been fighting for years finally beating him to a draw.

Tony Stark in the Hulk Buster, yes it took dropping a building on his head but he was able to defeat Hulk in a fight.

So within the MCU we have already seen Hulk is not as infinitely strong as his comic book alter ego. Now add to this the fact that Thanos has been defined as the most powerful being in the Universe, he has an infinity stone already, has been shown to beat Thor (who could hold his own against Hulk) and is therefore easily a match for Hulk. Remember Hulks power comes from Anger, now if Hulk is being beaten by Thanos fear and doubt starts to creep in. This then leads to these emotions overcoming his anger meaning he would actually weaken meaning Thanos can cause more damage leading to him weakening more.

This fear and shock at being so easily beaten would explain why Hulk does not want to come out to face Thanos again later in the movie.

This was discussed by the film's directors, the Russo Brothers in a recent interview. In short, Thanos is as strong as the Hulk and more than a match for him in terms of fighting technique. The only thing Hulk has going for him is raw aggression and that got him a slight advantage at the start of the fight along with the element of surprise.

Note also that Thanos wasn't using the Power Stone to "actively" augment his own natural abilities. Had he done so, presumably he'd have been able to punch a hole straight through the Hulk and out the other side.

I would say that he’s just that powerful. You didn’t see him actively use the power stone in that fight. I think ultimately, the way we looked at that fight and the way talked about it with our stunt team when we were executing it was Hulk is obviously very powerful, but he’s a little mindless in his fighting style. It’s aggressive; it’s pummeling. Thanos is the Ghengis Kahn of the universe; he’s a very skilled fighter, equally as strong. So when you put those two up against each other, the more skilled fighter is going to win ultimately. Which is why Hulk has a moment where he overpowers Thanos, but ultimately Thanos is smart.”

_{^{The Russo Brothers Share How Thanos Beat Hulk In ‘Avengers: Infinity War’}}

That all being said, in the Endgame Prelude comic, you can see that the power crystal is lit (at times) during the fight. Although Thanos may not be 'actively' using it, it's possible that he was passively using it and drawing strength from its mere presence.

_{^{Marvel's Avengers: Endgame Prelude (2018) #1}}

Idk seems I see a little "grey in the temples" on hulks buzz cut in IW when he fights Thanos, and I think it's totally plausible he got caught short expecting an easy win until Thanos peels Hulks' hands off and gives him a throat punch that shut him down quick... he seems to slow down an the rage is subdued there foreword by contrast to his opening, maybe that's just my perception, ymmv. as was said well before, it seems to me our Hulk wasn't quite prepared, at least for the amount of power in that punch and it seemed to take the fight out of him, hence the early (and much disliked but necessary loss) Movie plot necessity aside, naturally...

The MCU Hulk doesn't get stronger and more durable (that's how his powers actually work, it's not just strength). This has never been established. The MCU Hulk is a convenient plot device used to establish the power of other characters e.g. Stark's Hulkbuster armour and Thanos. In the comics, Hulk can trash even adamantium so no metal suit or robot would be safe. Hulk and Thanos have never fought one on one in the comics without any additional power ups. Hulk's strength feats dwarf anything Thanos has done in canon and given Drax has killed Thanos several times, Hulk shouldn't have too many difficulties but the Russo's aren't Hulk fans and there's little consistency in the movies (Cap managing to resist Thanos) so don't think too hard about it!

Well first of all Hulk's activity or lack of same in the rest of the movie indicates that he was something less than fully motivated and so does his role in Ragnarok where he starts indicating that he's resentful at being used by his Banner half and the Avengers. He may not have been able to muster all that much anger. Also his comic book exploits are irrelevant. That Hulk is a different character and one who had decades more of power escalation to make him stupid powerful. The characters of the MCU are roughly at Bronze Age of comics levels of power.