Hollywood loves franchises. Sometimes, frankly, they love them a bit too much.
Cinematic history is ripe with tales of movie franchises that were soaring on the wings of success, right up until the point where they pushed things one film too far. No matter how many great films a series may have under its belt, all it takes is one colossal misstep for the entire thing to go up in smoke.
You know these franchises: they, more often than not, begin with a true cinematic classic that captures the hearts of millions. And while that may be great initially, as the creators bask in the warm glow fandom, it can become a curse. While the first film may have somehow pulled off the incredible feat of acquiring fans, the sequels have the really hard part; keeping them satisfied.
Just as fans may love a franchise dearly, they will also hate a franchise even more deeply. Thus, if fans feel a sequel is not up-to-snuff, they will surely be vocal about it and if the creators aren't careful, they could very quickly be looking at an expired property.
These are the film franchises that tried desperately to keep their film series alive, by pushing one (or several) film(s) too far. Sometimes they lose their creative mojo, sometimes the producers are just trying to squeeze out every cent they can.