For anyone who needs a refresher on why this is an obnoxious question:
This is from a few years ago because FFN and Wattpad stats require you to take a random sample and hand-count. (In these charts, the multi & other category includes everything with multiple ship types listed in addition to things labeled as 'multi' and 'other'. That's because this is basically not a thing outside of AO3.)
The difficulty of grabbing other stats is why wankers always get to pretend AO3 is everything and m/m is the big category that must somehow be responsible for the lack of f/f.
People who ask this question are either too stupid or too malicious to be able to research fandom.
AO3 was founded by m/m shippers. The early adopters were m/m shippers.
It's like asking why the clubhouse has club members in it. I don't know, Susan. Truly, it is a mystery.
As time passes, AO3 gets less m/m-centric, but it's still a rare archive where you can filter out het, so it will always have more m/m (and, indeed, f/f) than many spaces.
And before someone bawwwws that it's ~the most popular site~
Go make another site popular...
Or post more of what you want and invite more writers of what you want to AO3.
FFN was more popular than AO3 for years. Even after its traffic began to decline, it had more works.
Wattpad was definitely just as popular (and big!) if not vastly moreso until recently. Whining that you personally don't like the site won't change that it's wildly, wildly popular.
You want to complain that the world is too straight but the queer cultural spaces tend not to be for f/f? Fair enough.
You want to complain that the BL section at the bookstore has BL in it while standing with your back to the TOWERING WALL that is the het romance section?
Fuck you, you homophobic, TERFy little shitstain.