In reponse buba, most 'sucky' sequels are made by Disney. the original films have been well-recieved and had become beloved by those that watched them. The stories were well-written, and the audience may have known the stories previously. Furthermore those stories stood by themselves for many years, no sequels were made to them. At a certain point after the original film, a sequel becomes unnecessary. the actors and actresses who played the characters can no longer play them in a sequel. In Walt Disney's time there were no sequels, but a fresh and different story. Sleeping beauty, Fantasia, Peter Pan etc, each one different. The first sequel to a Disney film was The Rescuers Down Under and that took place 13 years after the original film. It was good, not quite as good as the original, but when compared to more recent sequels it ranks near the top. The Sequels that are as good or better than the original film are few in number, those that are are either part of a long series, see LOTR, or the original was merely decent to begin with.
As for why are the sequels bad? First, most sequels do not get shown in theaters, but rather are direct-to-video releases. Lion King 2, Cinderella 2, Bambi 2 Pocohantas 2 etc. The few that do get put into theaters rarely do well. See Jungle Book 2 and Peter Pan 2. I had the misfortune of seeing Peter Pan 2, and it paled in comparison to the original.