We have no reason to think there's any stubbornness at all. Universal might well simply be indifferent to the matter. They aren't making a deal because they don't care, they have plenty of other business to handle and don't need to care about one old distribution contract. Disney meanwhile doesn't necessarily need to care, because the only thing that old contract prevents is something they may well not especially want to make anyway: a solo Hulk movie. Sure, it might be nice, but two solo Hulk movies in a row have fizzled, he's one of their more expensive characters to realize, and his traditional "iconic" form is one of the trickiest to actually put into a heroic character-based narrative ( doubly so with the problematic expectations of the loudest fans* ). With such a giant roster of heroes to use, and a proven ability to turn virtually anything into gold regardless of prior fame, it just doesn't matter that much.
* The classic, iconic Savage Hulk, in his widely recognized form? To put it bluntly, is the villain of the piece. He is a tragic monster, but a tragic monster is still a monster, and the tragedy is that Banner would be entirely good to be rid of the Hulk. The tragic hero of the tale is Banner, and the Hulk is the bad thing that happened that makes it a tragedy. Acknowledging this through the narrative. . . is a problem, when a good chunk of the fans do not accept that a rampaging destructive monster with the intelligence and self control of a child at best, is a bad thing. Instead treating the idea of power uncontrolled by anything, even the self, as the best most awesome power fantasy ever. It is indeed a power fantasy, but a villainous power fantasy, not a super heroic one. Now, can this issue be avoided? Sure, but you avoid it by making the Savage Hulk less, well, savage: by giving him personality and self control and intellect sufficient that he is capable of being a moral actor. This might mean becoming a merged Professor Hulk who is basically Banner With Powers, or it might just mean being a monster with enough self control and awareness to recognize friends and foes, and know which one to smash. However, any version of this means giving up at least some of the "uncontrolled" premise of the Savage Hulk, accepting that the character actually is restrained by moral boundaries.
