Many movie sequels suck. Not all of them, but once the movie has been made, they have to think up a plot for the next movie, which might not be as original.
Rocky XII- The Return of the Hobgoblins
Also, it seems that movies always try to "up the ante" when they make sequels. For example...
Jurassic Park- Had a T-Rex. Cool.
The Lost World- Had TWO T-Rexes.
Jurassic Park III- Had a Spinosaur. Woah.
Star Wars/EmpireStrikesBack/ReturnoftheJedi- Lightsabers.
The Phantom Menace- A DOUBLE-BLADED LIGHTSABER! Holy shit.
The Clone Wars- Anakin uses TWO lightsabers.
Revenge of the Sith- Grievous uses FOUR lightsabers. What an animal.