I can tell you why, in my case - it's no secret.
(1) 5e does nothing for me that 4e does not do better.
(2) 5e has no robust non-combat conflict resolution framework, which is pretty much a deal-breaker for me in my RPGing. (I've often posted that Classic Traveller has a similar weakness in its on-world exploration rules. It's not a coincidence that since I discovered this - via the process of actual play - that our game has featured no more on-world exploration.)
(3) The asymmetric resource suites in 5e mean that maintaining mechanical balance of effectiveness across the PCs depends upon multiple encounters per "adventuring day" - 6 to 8, or at least the prospect thereof. That makes the "adventuring day" rather than the immediate encounter/situation the basic unit of play. These days that's another deal-breaker for me.
This is true for me as well. 2 and 3 combined are definitely the biggest ones (the conflict-charged scene not being the exclusive locus of play), but there are others. Lack of GM constraint broadly and lack of focus on a very specific type of GMing overhead (creating thematic complications, obstacles and situation re-framing as the conflict-charged scene evolves and ends, and advocating as hard as they can for "team monster"). That intersects with the game not being overwhelmingly player-facing. Relative dearth of forced movement/mobility/terrain interaction/stunting/thematic and tactical monster capability/team monster synergy. Lack of Minion, Swarm, and extremely robust Defender capabilities makes it difficult to reproduce certain archetypal conflicts like an "escort quest" (eg you're protecting a little girl you found floating on a raft from a Kraken's tentacles and then ultimately the beast itself).
Like I say a lot on here, its the
holistic experience of
the sum of its parts. Individual pieces of a game mean very little in a vaccuum. Its how they emerge to create an
integrated experience at the table. Not even Strike(!), the most kindred game I've seen to 4e can reproduce the 4e experience (but it does a Mouse Guard/4e hack awesomely!...so basically play Star Wars with).
If I want a dungeon crawl experience, I'll either run Torchbearer or Moldvay Basic (which, in turn, are hugely different from one another).
If I want a tightly integrated (characters, crew, setting, antagonism, decision-points leading to integrated change in all the prior) fantasy sandbox, I'll run Blades.
5e can't do 4e, but it can do both of those things (just not as well as those 3 games).
What 5e does far, far better than all of these games above is the "Metaplot-heavy, GM as lead storyteller, flying by the seat of their pants (ignoring/altering rules/results as required), entertaining players as characters move through an adventure path."
Interestingly, its basically an inversion of that continuum above:
4e is absolutely awful at that last one directly above, its actually pretty solid at the Sandbox (but you have to know what you're doing and the hexes should be player-facing in level) but nowhere near as good as Blades, its actually ok as a Dungeon Crawler (but you have to know what you're doing...using the Disease Track as Torchbearer's equivalent of Light and "The Grind"; condition accruement) but nowhere near as good as Torchbearer or Moldvay. Obviously, its unparalleled at being 4e.
So, if I was recommending D&Dish game archetypes for newcomers I would say:
* Play 4e for high-octane, Action-Adventure, scene-based play with mythical tropes.
* Play Torchbearer or Moldvay Basic for Dungeon Crawls (whether you want gritty realism or a cartoon).
* Play Blades for Fantasy Sandboxing (and any of its hacks).
* Play 5e for GM as storyteller, fast and loose, Fantasy Adventure Path play.