So here's my guess. I'm not trans, and what I'm about to say is purely my observation and not based on data.
Folks born with male genitalia doing anything that is not cis and straight is frowned upon more than folks born with female genitalia doing the same.
Society finds two dudes kissing more revolting than two women kissing. It also doesn't help that folks who transition to become a woman after having undergone male puberty are much more recognizable as trans. Compare this to trans men, who aren't so easily identifiable as trans.
Hence, trans women have it worse interacting with society compared to trans men. Hence, they crave for a safe space more. Lemmy's instances tend to be these safe places. Hence, more trans women here than men.
And cuz weed is better. Source: me