It happens with any long-running and popular sequence of stories, whether it’s a film trilogy, a television show, or a series of novels. The final entry in the sequence could cure cancer, end world hunger, and bring about peace in our time, and people still wouldn’t like it.
I call it the Write-Your-Own-Ending effect. Fans of any long-running series – whatever the medium – are going to invest themselves heavily in their own ideas/speculations/opinions about how the series ought to end. Satisfying all of them at once is going to be impossible. Satisfying any one of them completely is going to be almost as hard, since there’s always going to be something left over that doesn’t please. (“He/she never thanked him/her for this/that/the other damned thing!” “Why wasn’t so-and-so part of the Big Group Hug at the end?” “The ending was all about Titular Hero! Joe/Jane Sidekick barely got a mention! That just goes to show that Titular Hero really is a jerk, just like all us Joe/Jane Sidekick fans were saying all along!” And so on and on.)
The longer the dedicated readers or viewers have been waiting for the conclusion, the stronger the effect is going to be. Because they will not have been waiting passively all that time — they will have been making their own ending in their heads while they waited. Some of them will have actually gone so far as to commit their endings to pixels or paper; but even those who don’t take that final step have still been thinking and speculating and developing their own opinions about how things ought to turn out. So when it comes time for them to evaluate the actual conclusion to the work, they’re not just going to be holding it up against the previously existing material to see how well it fits — they’re also going to be holding it up against their own internally-developed Best Possible Ending, and the further it deviates from that ending, the more unhappy they’re going to be.