Sequels are supposed to improve upon aspects of their originals, whilst hopefully delivering on what made said original worth playing in the first place.
There are plenty of examples of fantastic followups that did just that. Games like Ocarina of Time, Super Mario Galaxy 2, Uncharted 2, Halo 2 and so much more all managed to help cement their franchises as some of gaming's best.
This isn't the case for all sequels however. Despite best efforts, some subsequent entries simply can't match up to the lofty heights of the originals, and can often end up being downgraded experience overall. In the absolute worst case scenario it can kill a franchise outright, royally peeing off the fans and ensuring there's no real 'reason' to return in the future.
The following entries did just that, and though many still have legions of fans praying for an instalment that'll make everything okay again, it won't be coming any time soon...