The issue with Markdown is the syntax is pretty horrible. To create a link, it's [ title ] ( url ), which is incredibly unintuitive. I'm still never sure if italics are or * *. Heck, if I want a single newline, that's not possible. I am all for using a markup syntax; I just wish we had a better option than Markdown specifically.
Additionally, Medium is specifically, heavily concerned with "how it looks" as that is one of the main selling points of the blog software.