It isn't that the UFC doesn't want him, it's that he doesn't want the UFC.
That's not really the situation. Sure Josh isn't jumping through hoops like most fighters to kiss Zuffa ass. He likes to do the pro wrestling thing and not be tied down.
But Dana White does not like him. For legit reasons. Josh Barnett has been busted multiple times and in crucial moments. He tested positive as the UFC Champion and was stripped of the title. He also tested positive right before one of the biggest MMA fights ever in what would have been #1 vs #2 according to many rankings. Because of him an entire organization when under when he screwed up the main event.
Is that really someone you want to rely on to headline your cards? In his biggest moments, he has been caught cheating.