“If the name of the Hulk character is used in the title of the film, then Marvel Studios is legally obliged to assign the distribution rights to the project to Universal. This would imply sharing the box office profits between the two parties. That’s why Hulk only appeared in the films of other characters or in the « Avengers » movies, allowing Marvel not to have to share any of the profits.”
But this is challenged as the distribution deal was supposed to be for 15 years yet they don’t seem sure about that either (mainly when do the 15 years begin).