A chart illustrating all of the possible explanations people have suggested for the lack of femslash in my AO3 ship stats survey, and the ways in which they might interlink.
Note: Not all of these theories are necessarily true, but the descriptions below are written as if they are.
Lack of female characters.
It’s still unfortunately difficult to find a show with one interesting and complex female character in it, let alone two. Without the source material, fanfic writers have nowhere to start.
Lack of female relationships.
Women are rarely allowed to interact with each other in a meaningful way onscreen; even when they are in close proximity, Bechdel test passes are rare and long-term chemistry even rarer. F/F relationships just don’t have the spark that so many F/M and M/M couples do.
Lacks favoured dynamics.
There are some kinds of dynamic between characters – close friendships, rivals, etc. – which are more likely to be viewed as shippable. In the source material, these dynamics are far more likely to exist between male characters.
In the media, men are permitted to be invested solely in their career or calling, but women must want relationships. The vast majority of female characters have canonical heterosexual romances, which abruptly quash the chances of F/F ships becoming canon.
Straight women want hot guys.
The majority of fanfic authors are straight women who want to write about hot guys, the more the better. Female characters just don’t interest them.
Women in fandom judge female characters far more harshly out of internalised sexist beliefs, while male characters are placed on pedestals.
Too close to home.
Most fanfic authors are women, and writing fanfics about female characters forces them to confront issues such as body image, everyday sexism, etc. which they find painful. Writing about male characters is a form of escapism, and allows these issues to be handled in a less personal way.
No pre-existing audience.
There’s already an established group of M/M fans who are on the lookout for potential new ships, and provide an established audience even in the early days of a fandom. F/F ships have smaller fanbases and are slower to be picked up upon.
Less writer support.
There are fewer writing resources for starting out femslash writers; it’s harder for writers to seek advice, do the relevant research and develop their style.
The feedback left on F/F fics is more critical than that left on M/M fics, and more discouraging to starting out authors.
Harder to find fic.
Because F/F fics are rarer to begin with, potential writers are less likely to encounter fic for the pairing, making it harder to discover new F/F pairings or research ones they are already interested in.
A new movement.
F/F shipping didn’t take off until the 90’s. It’s several decades behind the slash movement, and still finding its feet.
Movement lost momentum.
Back in the heydays of Xena and Buffy, there was plenty of femslash to go around, but after those shows ended, there was nowhere for the fans to go. If you want F/F fic, you need to find an older archive.
AO3 is biased.
There’s plenty of F/F fic out there, but AO3 isn’t the place to find it. It’s still in its early days, and the initial pattern of adoption has led to the archive disproportionately favouring M/M shipping more than fandom in general.
Hyper-popular fandom bias.
There are some fandoms – BBC Sherlock, Supernatural, Teen Wolf etc. – which have far larger fan bases than is typical, often with ten or twenty times the usual number of fanworks on AO3. Because these are often focussed on one or two M/M pairings, they skew the total results in that direction.
AO3 is primarily a fic archive, although is does accept other kinds of fanwork. F/F shippers are less likely to write fic in comparison to other media, such as art or vids, leading to apparently smaller numbers of works on the archive.
Western media is biased.
F/F shipping is under-represented in the popular Western TV and film fandoms. There is much more in non-Western or non-English speaking fandom, but these tend to have much smaller fanbases on AO3.
No side pairings.
Canon (normally F/M) ships and popular fanon (normally M/M) ships are more likely to be included in a fic as side pairings. Many of the fics in their tags hardly feature the ship at all, giving the impression that the ship is much popular than it is in reality.