> Clearly we do need something like Markdown, but the existing attempts (including Mediawiki's "wikitext") all reveal that design a markdown syntax looks easy, but is actually extremely difficult to do well.

Related to that, there's also an obvious "network effect" at play here. Every Mediawiki uses "wikitext" so to use Mediawiki you learn "wikitext". As a user you don't want to have to learn more than one of these syntaxes so you use whatever is most common. I think that's the tautological answer to headlining question "Why Is Markdown Popular?" network effects, specifically "Markdown is increasingly popular because Markdown is already popular". It's used in enough places and has to be learned by enough people that generally people learn Markdown. Once they've learned Markdown they expect to use it everywhere else (and so Markdown spreads).

(I had a time where I preferred reStructuredText and its increased power and capability and internal consistency over Markdown. At this point because of network effects, I've decided to just focus on Markdown like "everyone else", though.)