Lots of movies don't deserve or warrant sequels but have gotten them anyway. The creator of Shrek openly admitted that he originally had no intention of making a sequel to his story, as the original Shrek story did not warrant a sequel, and that money was the main (if not only) motivating factor to he and Dreamworks making one. Was Revenge of the Nerds worthy of 3 sequels? Shell no. But they got them anyway because the original Nerds made enough money for the studio to consider a sequel to be feasible (mind you, Nerds III and IV were made for television).
It all breaks down to dollars and numbers: if a film makes a substantial profit, then there's likely to be a sequel. It's rarely (if ever) an issue of quality.