For me it's several reasons. One is just that I have my own homebrew world that I've run campaigns in for decades. So a fair amount of stuff is tied into my version of D&D lore.
But fundamentally? Because it does what I need well enough. I'm sure there are other games that do certain things better, but while I do a healthy mix of combat and RP, I don't want a lot of detailed rules outside of combat for example. I'm comfortable with the basic concepts and tropes. I don't need or want the insanity rules from Cthulhu, nor do I want to play a game with a relatively narrow genre focus like Vampire the Masquerade.
Add in the fact that it's easy to find games with AL, easy to find players for my home game. Advertise a D&D game and people have a general idea of what they're in for. There are great tools nowadays (DndBeyond is awesome) and a ton of resources.
Maybe someday I'll get tired of running a pseudo-medieval garbage can of fantasy that is D&D but I don't see that happening any time soon. D&D is far from perfect, but it gets out of my way well enough for me to tell a wide variety of stories.
Ultimately I think "better systems" are in the eye of the beholder. No other system has ever offered enough to entice me away.