IME the primary reason successful campaigns don't go to high level is because their story has been told. I've run three successful campaigns in 5E, ending at levels 18, 7, and 9 respectively. The first one was an epic tale told in 3 arcs, plus I had an additional one-shot callback adventure using those characters. The second one had a bit of an open ending, but while I could have taken it to a second arc and into high level, I felt the primary story of the characters had been told. The third was a self contained tale that had no more to be said for it, so pushing to high level was pointless. Most APs are written this way as well, with their story being completed before high level is achieved.
Unsuccessful campaigns don't reach high level for a variety of reasons, but IME typically it's because the DM just gets tired of it, or worse, suffering from burnout. I don't think the challenge of high level DMing 5E is quite the deterrent that the online community seems to think it is, but I'm sure it's a factor.
Unsuccessful campaigns don't reach high level for a variety of reasons, but IME typically it's because the DM just gets tired of it, or worse, suffering from burnout. I don't think the challenge of high level DMing 5E is quite the deterrent that the online community seems to think it is, but I'm sure it's a factor.