Wow, this is almost identical to my experience and the various campaigns we played!I don't assume it, but in my own experience it tends to happen that way.
Of the three long (10+ years) 1e-style campaigns I've played in or DMed, one sank around 8-9th level*, one sank around 9-10th level*, and one sank around 10-11th level*; and in that order. I'm hoping my current campaign can get to 12th!
* - approximate party level average. Some individual characters got a bit higher.
The two of those I ran sank for different reasons. The first one I'd plain got tired of, and the rules by then needed a pretty big overhaul. The second one was still quite viable, but I just ran out of good ideas as to what to run.
The first was my homebrew campaign, exclusively using self-written adventures which lasted until about 9th level, the second (during which we switched to 2e) we just played through the 'classic' official modules, using the Tomb of Horror as a campaign ending device after about level 11, and the third was a Dark Sun campaign that ended at about level 8.
Most campaigns I participated in as a player sunk due to tpks. I've never played a character higher than level 9.