I think there is definitely a certain amount of truth in that. My preference as both a player and DM is lower-key games as I mentioned earlier. I suppose for myself it goes from the level of fun-fantasy (say tier 2 and tier 3) to simply ludicrous superheroic stuff (tier 4 and epic). I've run and played in epic level stuff, but at this point in my life it just seems meh.
The issue is more:
If it is mostly DM-influenced, why is that? and what about it bothers DMs?
I haven't run high level 5ed, so can't speak from personal experience. But my guess is that it is a combination of three factors:
1) Greater complexity of game play - and more to keep track of.
2) A general preference for 'classic" D&D--levels 1-10ish, especially the "sweet spot" of tier 2 (5-10), if only because of familiarity.
3) Unwillingness/inability to shift paradigms to "superheroic" campaigns.
The first two are pretty self-explanatory, but the latter might be where there is some wiggle room, and where WotC could help out with some kind of higher level guidebook and adventures--meaning, a supplement with recommendations on how to run high level campaigns, different adventure seeds, campaign structures and, of course, actually adventures.
I think part of the problem, which relates to 2 and 3, is that higher level campaigns require a paradigm shift. You can still play classic adventuring styles with more powerful monsters and bigger piles of loot, but why not explore different styles of campaign? World-shaking events, artifacts, leading nations, exploring the planes, etc.
I think 3-10 will always be the sweet-spot for most, but I see no reason why--especially with the "flattened curve" of 5E--the higher levels can't also offer tons of great play experiences, especially if the DM and players are interested in exploring different types of campaigns. And as I said, WotC could help the situation out by offering more high level campaigns--even ones that start at 11th level, rather than most ending in the 11-15 range.
