Markdown stresses ease of readability. I don't find the language low quality at all, just limited. Limited isn't necessarily bad, depending on its purpose.
> isn't GitHub the only reason why Markdown gets pushed so much?
Reddit's use seems to significantly predate Github, and I would say reaches a far greater audience. Unique users visiting in April 2017 is 1.285 billion[1]. Some fraction of that is probably unique people (given anonymous desktop and mobile usage), but given how large the number is, I imagine it's still a very large number.
> If GitHub chose AsciiDoc instead of Markdown as their base, we would be in a better position now.
I agree that Github would be in a better place now, but I don't think that would really have changed anything for anyone else. Even if you want to make a case that Github would have influenced programmers who would have then used it in other projects, I think you need to account for Stack Overflow also. I think it was arguably much more popular and used by a far wider audience than Github for a long time (and may still be, I'm not sure).
Markdown was used widely because it was simple, and users would actually bother to learn and remember the very few options they had. People are more used to it now, and at this point, sure, they might accept something more complex (especially if it built on the rules they already internalized), but I don't think we can blindly assume they would have accepted something more complex.
1: https://www.statista.com/statistics/443332/reddit-monthly-vi...
Edit: s/Unique IPs/Unique users/