And I think one way of looking at it isn't why not D&D, it's ... why D&D?
D&D is the default for a reason.
I think we covered that, above. Mad market dominance. Sole name recognition. Current version in print still recognizable as the same game that started it all 45 years ago.
It's easy in so many ways. You can't play TTRPGs and not have played D&D at one time (pretty sure about that!).
That's not easy, that's familiar. I mean, it's absolutely true: If you're in the hobby, chances are you got into it through D&D, and have mastered, or at least learned to tolerate, it's many eccentricities and shortcomings, and it's rules are second nature to you. They seem easy, because you've invested a lot of energy in getting them down pat - probably when you were young & had that energy to spare.

Now, not s'much ("learn another system? Ugh, they're all so complicated, I'd rather take a nap, then maybe play some D&D when I have more energy...")
(Yeah, I'm a cranky old man, but at least I admit it.)
But, no, it's NOT easy. If it were, we wouldn't always be shoving the (Champion) Fighter at the newbie because it's the simplest (sub)class, they'd be able to play whatever concept appealed to them.
New players have an instinctive understanding of at least some of the tropes of D&D (fireball! elves!). When it comes to parents, many of them want their kids to learn "D&D" because they have vague memories of the smart kids back in the day playing it (go figure, right? it's like SAT prep now). It's just ... you know ... there. And it's easy to discuss with other people- I mean, try and strike up a conversation about Amber (the diceless RPG) or Boot Hill or Torg or Living Steel or any of countless other games, and you might get a few takers, but talk about D&D, and the wall come a tumblin' down.
But D&D is, by its very nature as the market-dominant force, not for everyone. It can't be niche.
I mean, it /is/ for everyone, in the that market-dominant force sense, so can't be niche. Yet it /is/ actually kinda niche - it's one (sub)genre, done one, rather unique, way with about the same mechanics (class/level, daily spells, hps, deflection-only armor, roll that d20 to hit, roll that d8 for your longsword's damage) it's always used. But it's the hobby's gatekeeper, so it's niche /is/ the hobby, we're barely aware of the range of possible RPGs excluded from consideration because they wouldn't be enough like D&D to be recognized & accepted /as/ RPGs. (Heck, we see some people publish those, and hear "but that's not really an RPG is it?")
To be fair, it's pretty idiosyncratic (thanks, EGG!) for something so dominant, but still ... it has to continue to appeal to a broad swath of people.
Which it does, just not through it's idiosyncracies, but through name recognition, market appeal - and, I guess we could say, by virtue of being indoctrinated into it before you can uncover the more isolated corners of the hobby...