Since I'm in magical girl fandoms, you'd think I'd be shipping more F/F openly. Actually I write it in secret because I know it'd cause drama to ship any tomboyish character with another girl, because then you're lesbophobic and think all tomboys are lesbians by default. If you ship any super girly character with another girl you're fetishizing femininity and turning it into kink. Ship the rich girl with anyone and you're romanticizing wealth inequality or if she's a sad rich girl, you're saying abuse victims need romance to be whole. If you ship the short girls with anyone, cue pedophilia accusations. If you ship a girl and her best friend it's incest/they're like sisters. If you ship a girl and the redeemed villain girl you're pro-abuse. If you ship her with her civilian friend who doesn't know her secret identity you're pro-manipulation. Ship her with the main character and you're saying romance is mandatory for heroines/treating romance as a reward for being a good person. Ship two non-mains and you're clearly lesbophobic for not depicting the main lead as a lesbian. Depict the main lead as a lesbian and you're biphobic. Center your story on a bi girl, and you're lesbophobic.
I feel like when I post F/F I'm showing up at the playground and now I have to calm down the angry kids, except that metaphor breaks down because these people will dig through your blog to find things to make a callout post about you for. And while I like real-life kids, trying to babysit these children is such a hassle.
The magical girl fandom really acts like this and then wonders why there's not more F/F out there. IDK, maybe because the first time I posted Sailor Mercury/Sailor Pluto I got accused of fetishizing the Romani? (for those unaware, Takeuchi stated Pluto is Romani. It's a long story) Maybe because when I wrote Homura/Hitomi I was accused of being butchphobic? Maybe because the last time I reblogged a moodboard for a crossover magical girl couple I got called a pedophile because one of the two 14 year olds is 5 inches shorter and that's treated as a crime?
Even if the cast is majority women, if the fans are shitty, other fans will go, "eh. I think I'll go make fanworks for this much less shitty, much less obnoxious fandom over here" and the result is a lot less likely to be F/F just due to statistics about the genders of the cast.
The "why won't more people write F/F?" question is basically, "why won't anyone get into this bed with me after I shit in it?"