Rich text editing on the web is complex, Markdown is simple. Adding rich editing for something as simple as a comment box typically means adding something like

Rich text editing on the web is complex, Markdown is simple. Adding rich editing for something as simple as a comment box typically means adding something like TinyMCE: half a meg of JS to build the edit UI and attempt to paper over browser quirks and edge cases. Markdown is just a <textarea> plus a call to your renderer on the back end. It's offloading complexity to the user.