Why did the Tesseract teleport Red Skull to Vormir, and why was he forced to be the guardian of the Soul Stone?

Why did it transport him?

Red Skull himself explains to Thanos and Gamora why he was sent to Vormir instead of being able to wield it. The Space Stone cast him out and forced him to be the Guardian.

Red Skull: A lifetime ago, I, too, sought the stones. I even held one in my hand. But it cast me out, banished me here. Guiding others to a treasure I cannot possess.

_{Avengers: Infinity War}

Whilst it is never explicitly said this appears to be because of his abuse over the Space Stone. As such he was punished to be the guardian of the Soul Stone. This is somewhat back up in Marvel's Avengers: Infinity War - The Art of the Movie where Rodney Fuentebella, concept artist, states:

"We learned pretty early on that Red Skull was going to be returning," Fuentebella says. "I was so excited to work on this character that has been pivotal in the MCU. I was playing with how early the audience should know that this character-previously called the Stonekeeper in the script-is the Red Skull. I designed versions hiding his face and version showing how much life or soul the stone took from him. I wanted to play with the idea of what the Soul Stone had done to him and whether he is a ghostlike being now or something soulless."

_{Marvel's Avengers: Infinity War - The Art of the Movie}

Here Fuentebella is saying that there is further punishment from the Stones to Red Skull. Why else would it punish him if not for his abuse over the Space Stone? It doesn't know of anything else. Therefore, it seems most likely that the Space Stone sent Red Skull to be the Stonekeeper for the Soul Stone for this reason.

Other characters have held the Tesseract/Space Stone with their bare hands and not been sent to Vormir like Red Skull. However, there are two likely factors as to why that was the case:

1. They hadn't abused the Space Stone before touching it; when we see Thanos hold it that is the first time he has come into contact with it that we know of.
2. The Soul Stone already had its Stonekeeper; it's possible that the abuse plus the Soul Stone not having a keeper combined is what led Red Skull to being teleported there. When we see other characters touch it, Red Skull is already the Stonekeeper and so there isn't need for another.

Note that in the Chinese Q/A with Joe Russo he was asked about Cap meeting Red Skull when he took the Stones back. He says that Red Skull isn't really Red Skull anymore but more of a ghost/a completely different entity now. The Infinity Stones have punished him for his actions over the Space Stone.

Q: How would Cap react when he encounter Red Skull when he returned the stone?

A: Red Skull would probably put the soul stone back to its location, and wait for the next unfortunate stone seeker to make sacrifice. Cap and Red Skull probably won't fight. It's because it's his mission to return the stone to its original place. The Red Skull is also no longer the same Red Skull from FA. He is more like a ghost, you could almost say he's a completely different entity now. He only exists to guard the stone, his past conscious may or may not exist anymore.

How credible is this?

I just want to add in here something that came up in the comments. We see the Power Stone has an affect on people who touch it, if they are not powerful enough to control and wield it, it will consume them. The most notable example of this is of course Carina in Guardians of the Galaxy and then the Guardians themselves linked together at the end of the film.

One can draw parallels between the Power Stone and the Space Stone here in that maybe there is some requirement to wield the Space Stone and Red Skull didn't meet it. It's speculative and not clear what the requirement is though.

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I just also want to throw another theory I've seen thrown around before in here. I'm of the opinion that it isn't as credible because it doesn't match exactly with what we see in Avengers: Infinity War and Avengers: Endgame but it is worth mentioning.

The theory goes that the Soul Stone simply teleported Red Skull to Vormir. Once there Red Skull himself works out the Soul Stone is there and that he needs to do a sacrifice to get it; how isn't explained. The theory states further that because Red Skull loves himself he sacrifices himself to get the Soul Stone hoping as it is the Soul Stone he can save himself from death. However, the Soul Stone then imprisons him to be its keeper.

It doesn't make complete sense but it is worthy of a mention.

Why Vormir?

I think the most likely reason it chose to send Red Skull to Vormir instead of anywhere else is because of the special place the Soul Stone holds. It's not just hard to find, it requires more than just you being able to wield it to be able to possess it. You need to sacrifice someone you love at the monument on Vormir to be able to get it; that needs instructions and what better way to do so than with a Stonekeeper?

Thanos: What's this?

Red Skull: The price. Soul hold a special place among the Infinity Stones. You might say it has a certain wisdom.

Thanos: Tell me what it needs.

Red Skull: To ensure that whoever possesses it... understands its power.... The stone demands a sacrifice.

Thanos: Of what?

Red Skull: In order to take the stone you must lose that which you love. A soul... for a soul.

_{Avengers: Infinity War}

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Out of universe

I just want to add in here that the out of universe reason has been given as to why they chose Red Skull. In an interview with Collider Joe Russo states they chose him because it was someone the audience could trust.

“I think he was always that character. I think we may have played around once or twice in it not being him, but we needed someone the audience would trust, so that when he explained the rules of how you obtain the stone, that they would believe the character. I think it was Markus and McFeely who fought really hard to have him back in the film..”

_{[Collider, Exclusive: ‘Infinity War’ Directors Explain How Thanos Wields the Soul Stone; Red Skull’s Connection]}

This idea is reiterated in the "Art of" books as well as mentioning that it was because they wanted to explore "where" he went.

On Vormir, Thanos and Gamora encounter the keeper of the Soul Stone: the Red Skull. "Cinematically, he's the first one ever to be obsessed with these Infinity Stones in the MCU," Co-Writer Christopher Markus says. "And he clearly did not die at the end of Captain America: The First Avenger. The idea of where he might have gone was so tantalizing, and so we had a character who fit this moment perfectly."

_{Marvel's Avengers: Infinity War - The Art of the Movie}

In Avengers: Infinity War, the Red Skull tells Thanos and Gamora that:

Red Skull: A lifetime ago, I, too, sought the stones. I even held one in my hand. But it cast me out, banished me here. Guiding others to a treasure I cannot possess.

_{Avengers: Infinity War}

In addition, looking back at how the Space Stone (Tesseract), it always has been shown to teleport along with its wielders: Loki (during Avengers) and Thanos (during Avengers: IW). However, in case of the Red Skull, the stone sent him to Vormir but it stayed back on Earth.

Speculation (on my part): The way Red Skull misused the Tesseract's power; the Red Skull wasn't teleported/transported; he was most likely banished to Vormir.

As mentioned in the wiki page (apologies for basing it on a unsourced wiki):

Red Skull found himself teleported into Vormir, the home of the Soul Stone, punished for abusing the Space Stone's power. Red Skull was cursed into becoming the guardian of the Soul Stone for over seven decades, rendering him unable to leave the planet throughout these years, only able to advise those who came onto the planet seeking the Soul Stone.

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Why Vormir?

As the Red Skull explains in Avengers: Infinity War,

Red Skull: The price. Soul holds a special place among the Infinity Stones. You might say it has a certain wisdom.

Thanos: Tell me what it needs.

Red Skull: To ensure that whoever possesses it... understands its power.... The stone demands a sacrifice.

_{Avengers: Infinity War}

The Soul Stone, being the "special" one amongst all the Infinity Stones could have been some sort of a beacon.