Reasons I've experienced, and seen others cite:I remain deeply interested in the answer to the question of whether most games don't reach higher levels because they prefer not to play at those levels, or whether most groups don't get there because of time.
There is also the lack of published support, but that's a bit chicken and egg.
I will say that the vast most of the campaigns that have run for more then a year have not been with D&D but instead in systems with less steep power curves.
Group breaks up.
- Players move away.
- Players no longer have time due to family, jobs, etc.
- DM burns out.
- They find combat gets too bogged down.
- They get bored of their PCs or the campaign.
- They want to change to try a different RPG or switch to boardgames for a while.
- They find the combat gets too bogged down.
- Find running the game at high levels onerous.
- Find it difficult to challenge high level PCs.
- Get bored of the campaign and want to try another one they've bought or created (Seth Skorkowski talks about this in his videos).
Which ties into the point of DMs wrapping up a campaign once they find it difficult to challenge PCs. For me, D&D loses much of its appeal if perma-death isn't on the table as a source of tension.