The studios can't seem to agree on anything.
Actor Mark Ruffalo (who currently plays Doctor Banner) commented on the behind-the-scenes bickering that's keeping another solo Hulk movie from happening (via):
“Marvel and Universal famously don’t get along very well, and so that’s working against us, definitely. But I know for a fact that everyone is holding out hope that one day we can do it. But the nature of the relationship right now, it’s a little prohibitive... right now it doesn’t look particularly promising."
Because Universal still holds the distribution rights for the character, Marvel can feature him in the ensemble movies but they're not able to give the character a solo film. And in the meantime fans are feeling, well...
It's not really fair that some studio disagreements are keeping this beloved character out of the hands of those that really love him. On a scale of pink to very green, how angry are you?