1. We get a _lot_ of casual contributors to MDN: about 180-200 unique contributors/month, most of whom we never see again. Almost all of them can contribute much more easily with Markdown than with anything else. Many of these people are unlikely to put even an half an hour into learning a new syntax.
2. Markdown has great tooling support. For example, if we want to run Prettier over our embedded code samples, it's really easy if we are in Markdown. If we are in Markdown we will get nice formatting just about everywhere, including GitHub of course and most people's editors.
One thing that made the choice easier for us is that MDN's a very mature doc site, so we had a very good idea of which Markdown limitations were likely to be a problem for us.
If you're interested, we blogged about this project: https://openwebdocs.org/content/posts/markdown-conversion/.