Because Universal Pictures still controls the distribution rights to Hulk films, The Incredible Hulk never received a sequel, and Mark Ruffalo’s version of the character has instead been a supporting player across the Avengers movies and Thor: Ragnarok.
The mind of the former in the body of the latter produced “the best of both worlds.”
That’s not to say there hasn’t been any character development for him though, and it all finally paid off in Avengers: Endgame when Bruce Banner and Hulk merged into one being.
Hulk has certainly been on an interesting MCU journey.