I think a major reason, especially these days, is because sequels are often motivated purely by money, and not by having more story to tell. Any movie that does well has to have a sequel now, so you end up with sequels devoid of new ideas, because they just want a movie to appeal to the same audience again. If it worked once, they figure it will work again. Never mind that it's just empty rehash.
Barring sequels that are just crap, I think it's a matter of expectations too. For the first movie, you have no preconceptions, because you don't know what you're getting yet. A sequel has expectations and everyone has a different idea of what direction the sequel should take. I don't think you can win.
Just look at Terminator 2, one of the best sequels of all time. There are still people who don't like it that much, because Arnie is a good guy, and it's not gritty enough. You can't please everyone.