Not all sequel sucks, but only very few is better then the first one. i.e. I consider Aliens and Terminator 2 as very good sequels that are different, yet has the same quality then the first ones.
Usually the quick money scheme is the primary reason. The problem is the quality / quantity stuff. Major studios believes the name itself is garanteeing the success. So they're accepting a low quality script in most cases, adding dozens of CGI scenes to have a chance to add good scenes into the advertisments and... that's all (I think here for fantasy and sci-fis.). As for other genre (Thrillers, horrors.)... sometimes the the same quality and success cannot be achieved in a sequel, because the story was developed only for one movie, not for a sequel, trilogy or quadrilogy.
If the studios would take a bit more time to take a look at the script itself, the sequels could be good enough. Unfortunately in the movie world, where in most cases the explosions, senseless CGI parades are dominating, scripts are just secondary or worse, tertiary elements in the eyes of the studios.
And the last problem, studios are not famous about daring to create new ideas, rather creating remakes, disguised as sequels. Same jokes, same setup, etc, etc...
But as I said, everything would depend on the script, yet when the script is a tertiary element, don't count for a miracle in the sequels or sometimes in the original products.
Movie 1 - OMG! it's so good, best thing ever! Gimmee Moarrrrrrrr!
Movie 2 - Quick! Write another one before they get over the wave from the 1st one! The actors are under contract, they don't have a choice! $$$$$$$
But Cyia's points summarized this completely.