DND_Reborn
The High Aldwin
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.
Well, having played through and into the Immortals Set (can't really get much more powerful than god), changing paradigms is pretty much needed.
I agree it would be great of WotC to develop tier 4 and epic level adventures, to support those levels of play. A lot of DMs do avoid them IMO because the challenging the power of PCs at that point make is difficult.
For myself anyway, I like playing characters that are ordinary people who do extraordinary things, but frankly have no desire to do superhero things anymore. I've been to Hell, the Abyss, and saved the world from apocalypses, created artifacts, destroyed artifacts, and conquered nations. Of course, that makes it easier for me if I want to run such an adventure...