It's been pretty well documented that it was because he had already introduced Chewie as a character that can fly a spaceship and have intelligence and that Wookiees could no longer be the "primitive" race to defeat the Empire that George wanted.

I agree, that was one of the main reasons too.

Even Gary Kurtz said that he became disillusioned with George after ESB when he noticed that his priorities had changed from Story and character and focused more on selling toys.

Well not "even". Kurtz was pretty much the one who came up with that crap. It was in that interview where he tried to convince people that he left because Lucas changed his priorities... while in reality he got fired because ESB spiralled over-budget.