1-They've already made two Movies, and they weren't very good
2- The last one didn't really make that much money
3- Marvel already pulled the plug on a sequel
4- He makes The Avengers movies more special
5- The character just works better on a team
6- He's actually kinda getting his own movie anyway (it's just split across three other movies)
7- Keeping the Hulk onscreen for two hours isn't cheap, and isn't easy
8- Hulk's greatest battles are internal, not external
9- There are some distribution rights issues ,Yes, Universal apparently does have right of first refusal to distribute a potential new Hulk film—which could create some problems
10- The good Hulk stories are typically pretty weird
11- There's not really anywhere to put it on the schedule