Why couldn't Kai just come back again?

Question

In Kung Fu Panda 3, the character of Kai

1. is sent to the spirit realm by Oogway

2. is able to return to the mortal realm once he'd stolen enough chi

3. Po is unable to use the Wuxi finger hold to destroy Kai as Kai is a spirit being. Po then wraps his arms around Kai and uses the move on himself, sending them both to the spirit realm!

Upon arriving there, Kai shouts something like "No, not here again!?!" as if he were trapped there. My question is, couldn't he simply

return to the mortal realm again, since he still has all the chi he stole like before? Nothing's changed. He could just leave Po there, right?

Answer 1

For those wishing to avoid spoilers, don't play the videos linked here.

There's a few reasons for Kai just not immediately leaving. The end of this video from the very beginning of the movie sets the initial context.

We see two things here.

1. Oogway challenges Kai by saying that he "sent another" to stop Kai. Kai immediately vows to find him and take his chi as well.

2. The move to transport from the spirit world back to the real world appears to take several seconds. Unless Kai has defeated or otherwise delayed his opponent for several seconds, he'd probably be vulnerable to an attack which, I'm reasonably assuming, would interrupt the transport.

Next, we have this scene

Here, Kai is reaffirming his promise that he will take Po's chi.

Later on, we have the scene you're asking about.

Kai specifically says, "You brought me back!?" From his tone, I don't really get the feeling that he feels trapped, but that he's simply really annoyed. Po makes a lame attempt to placate Kai, who responds angrily that they'll finish their fight there in the spirit world. This probably puts Kai on tilt a little bit.

But,

the real reason is that Kai wants more power and Po's chi will give him that, and it's not clear if Kai would have had an opportunity to return until after he defeated Po and taken his chi.

Answer 2

I think it was because Kai needed a great amount of power from other's chi to move from the Spirit realm to the Mortal realm. That is why he needed to take Po's chi so that he could return to the mortal realm and take the chi of the other pandas.Like he said at first, Master Oogway's strong chi was needed for Kai to move to the Mortal Realm. Later Kai needed Po's strong chi to again go to the Mortal Realm. That could be the main reason.