Sequel of a movie...

Because usually they are just doing a rush job to cash in on the success of the first one.There are exceptions,such Aliens with to me was better than Alien.

They don't take the time to write a good script like the first movie had. They know that by just using the name of the first movie and several of the same characters they'll attract at the box office based on the success of the 1st movie. They make money producing junk just like some mylotters do by posting junk.