I don't need to use the mouse with Markdown. The keyboard is sufficient, and once you have a bit of practice it's easy and quick to use.

Additionally, I think it emphasizes the styles that are important in social media (blockquotes, code, bold, and italics) while de-emphasizing those features that are less important to it. The rich text editors, even on the web, all have a standard layout of buttons which displays all the wrong features first.