To repeat my earlier point - Marvel Studios seems to believe Hulk works best in a group dynamic, rather than as a solo character, despite that being his longtime mode in the comics. The monster success of Avengers only seemed to convince Marvel that Hulk is at his best with the other Avengers as contrasting characters in the same story. Besides, the holy trinity of the Avengers is Cap, Thor and Iron Man; Hulk getting another movie in 2008 was about reworking his origin to fit into the new shared universe and wiping away what happened in 2003. If he took off at the box office, maybe things would be different. The fact the film was only a middling success, was certainly offset by Iron Man's unexpected monster success. Minimum goals were accomplished with TIH in 2008. Now Marvel has it's sights set on expanding other important corners of their universe.