Because much as I love indie RPGs, they tend to require particular players with specific expectations, creating a sort of fragility in play. If you don't play the sort of game the author intended, the game feels boring and unsatisfying.
D&D and its family, OTOH, create an immediately familiar environment and set of expectations. Even if you've never played it, you understand exploring a ruin and fighting monsters.
Not that I want to do that, either. But it's a robust concept.
@BrentNewhall I think some of that "fragility" is in lack of tools and clarity of purpose. I noticed when play-testing Haunted that I had to do a lot of "guiding" and I wanted to reduce that.
So I specifically implemented tools to reduce that need. The game's GMless so there's a certain fragility that comes with that but the tools really really help keep play focused.