Well, what movies are block busters and what movies are not seems to say more about the audience for movies than about what Hollywood is doing, I think. Hollywood prefers to make movies that get audience. But not all movies have to be block buster to be successful.
It is kinda amazing that there is even a way to spend a hundred million dollar budget on a movie and be financially successful. Now, you could ask why such kind of movies are typically action and sci-fi movies, but is that really surprising? Do you need Dolby Surround Digital Super-Deluxe Elite Edition and more importantly a gigantic screen to enjoy a comedy with witty dialogues? The extra immersion that the cinema features brings you is something that not all movies really need. And that's why they don't get to become blockbusters so easily, because you don't really miss a huge part of the experience if you watch them at home.