Echoing comments that not all campaigns have a planned ending to start with.
As a matter of fact, campaigns having a planned ending is still a relatively new concept to me. When I started playing, a “campaign” was a series of adventures including the same characters or at least, set in the same chronological setting. Only since 5e did it become clear to me that a single adventure book, like Curse of Strahd for example, was considered a full campaign and the expected end of for these characters. Perhaps we never played (A)D&D as intended…
So campaigns fizzling out because life happens was the expected outcome. The DM saying “Good job folks, you won! What should we play next?” was a very alien concept to us, or one reserved for a few specific RPG such as Call of Cthulhu or Paranoia (the first because it wasn’t expected for your characters to have the sanity to keep going, and the latter because we didn’t want to commit to a parody RPG even if campaign-as-series-of-adventures was the intended playstyle).
So back on t he subject; my experience with campaigns ending early…
- Life happens. People move, fight, split, die, get deployed, have kids, get time-consuming jobs, and scheduling becomes impossible.
- Look, shinny! Players and GMs get attracted to try new games, play in a different era of the same setting, have bright new ideas of things they’d like to try, etc.
- My turn now! Player wants to try DMing, or DM gets tired of DMing.
- It ain’t like it used to be. People grow in different directions and friendships erode over time.