Because it's boring.
I won't say it isn't a well executed game. It's simple, streamlined and straightforward, combining elements of the classic experience with a new player friendly way of operating.
But... It's completely without novelty. There is nothing new in 5e. No innovation, no progression, no new ideas of any kind.
I can see why new players, or D&D purists, love it. It's a modernised iteration of everything they love about the game, combining familiarity with a lot better execution than classic forms of the game ever had before.
But... For me? For people I play with, who play a lot of other games? There's no reason for any of us to care about 5e. Anything it can possibly offer, another game has already done, done better and done in a more interesting way.
5e is a game which might have expanded the scope of D&D, but in many ways it's kind of irrelevant to the rest of the RPG industry- Admittedly, the rest of the RPG industry is significantly smaller than D&D and its offshoots.
But yeah, that's basically it from me. I'm well aware I'm not in their target audience, but I still expected better from them, especially after stuff in the playtest.
For all 4e's faults and controversies, it tried something new and was utterly astounding as an example of clear layouts, easy to use templates and good, user friendly rulebook design, as well as the best game out there for grid based tactical combat.
For basically every edition of D&D you can point to some aspect of it that was new or interesting or meaningful, that had an impact and opened up new design possibilities. 5e... At least from what I've seen, and after a few years of play and discussion, just doesn't. Its resting on its laurels, and that disappoints me.