It’s tough but one of the tenets of Gemini is that a lone programmer can write their own client in a spirited afternoon/weekend. Markdown is just a little too hard to clear the bar. Already there was much bellyaching on the mailing list about forcing dependence on SSL libraries; suggesting people rely on more libraries would have been a non-starter

Note that the Gemini protocol is just a way of moving bytes around; nothing stops you from sending Markdown if you want (and at least some clients will render it - same with inline images).