The only thing I can think is that there was a feeling Markdown was better for mobile (which we now know for sure was the motive behind the site redesign), and there's no WSYWIG editor component that produces Markdown. I'm not sure that's true, though, so my supposition there is probably suspect.

Other forums I frequent tend to use BBCode posting with a WYSIWIG editor on desktop platforms, but you have to use BBCode if you post from mobile. Typing BBCode with a software keyboard is kind of sucky, which is why I think Markdown isn't going to be much improvement, because it uses some of the same characters, which aren't super easy to type on a software keyboard.

Markdown might be easier than raw BBCode (on mobile) for some of the common, simple stuff, like italicizing or bolding a word. I don't think it would be better for something like entering a URL or a bullet list, which takes a lot of special characters.