Markdown allows easy copy/paste from Github, Reddit, and other modern sites.
The bbcode editor used here parses images as thumbnails immediately, which messes up editing on mobile, at least iOS, and messes up copy/paste. Markdown doesn't have that problem. In fact bbcode is a mess when it comes to copy/paste, because of the wysiwyg editing that hides/obscures most of the formatting commands.
Markdown is standard and used on many platforms. BBcode is slightly different in every forum software it seems.
Markdown is clean, much easier to write than bbcode, and also easier to read in source form.