There used to be a time when sequels were the exception, and not the rule. In the past actors and directors generally liked to move on and create fresh stories and characters, and outside of the promise of a huge check, sequels held little appeal.
Times change, however, and the era of the franchise is firmly in place. Now most movies, especially big budget ones, leave the door wide open for sequels, prequels, spin-offs and everything in-between. Inevitably, not every movie will be a hit, so it can be slightly embarrassing when the first part in a potential series ends (or is even titled) with a clear set-up for more adventures, and the audience just isn't interested.
