Why did Qui-Gon die immediately after a stomach wound?

According to Wookieepedia, Qui-Gon is human, so it's not that. Now then, I watched the the scene again to check where he got stabbed. It took him about 2:30 to die, which is actually fairly long. My first thought was that the stab might have been high enough to nick his heart. It certainly wasn't down in his intestines. Then it occurred to me that the strike severed his spine. But it's low enough in the spine that that alone probably wouldn't have killed him directly. Finally, I thought it would have severed his diaphragm. But if so, he would probably have expired sooner, and either way he wouldn't be able to implore Obi-Wan to teach Anakin. But I'm quite certain it wasn't below the diaphragm, so I have to say that the blade nicked his heart (and lungs and spine), not enough to kill him instantly, but enough to kill him after two and a half minutes.

There wouldn't have been much blood because Phantom Menace was a PG movie. Plus we've seen that the extreme heat of a lightsaber blade cauterizes wounds.

Still, looking at a circulatory system chart I see that the superior mesenteric artery runs right down the middle of the torso and feeds everything below the rib cage, which means it carries a lot of blood. Put a hole in that and you would bleed out into your belly in a hurry, even if there isn't a hole leading outside the body. Also, if a lightsaber is hot enough to cauterize wounds, it's plenty hot enough to boil bodily fluids. Darth Maul left the saber blade in Qui-Gon a few seconds before pulling it out, so besides the carbonized guts we can add live steam parboiling Qui-Gon's innards. Imagine someone taking a white-hot billet of steel out of a furnace and ramming it in one side of your body and out the other. The wonder of it is that Qui-Gon lived for as long as he did. Consider what happened to solid metal doors when Qui-Gon rammed his lightsaber into them:

The fact of how he met his end has been established. I think we should also answer why it took him so long to die. Qui-Gon was a reputed practitioner of the breath control technique. The idea being you can lower your body's need for oxygen and so forth (by extension they can also control their biological functions. Heart rate and so forth). Presumably, Qui-Gon used this technique so he could survive long enough to deliver his final request to Obi-Wan. Reputedly some masters had perfected the technique so they could survive days without air. Unfortunately as the books demonstrated, Qui-Gon wasn't so skilled.

As a medic, I can say that his death would have been swift. At least it's true that he wouldn't have been awake long.

The area where he was stabbed contains the pulmonary arteries and you won't live long without blood making it to the heart. Moreover, the damage done to the heart and lungs by being in proximity to such heat would cause severe damage.