I suspect a lot of it's inertia from before the choice mattered or was actually reflected on (though I guess there are still plenty of projects changing).

It's easy, most projects can satisfice with it, and people on the projects that can't satisfice with it may not think about markup enough to realize they're painting themselves into a corner until they have a big ballast of existing documentation to cope with?

I've been fumbling around for how to convey signs that a project may need better tools (https://t-ravis.com/post/doc/what_color_is_your_markup/) but it's been slow-going and I'm bearish on how well ~better-practices will spread.