I'm really interested though, and still don't quite understand, how it so overwhelmingly became straight girls writing canonically straight male characters in gay relationships (or their mental image thereof, which is very unlike gay male sexuality). There's something quite weird happening there.
This is a big complicated answer with a lot of factors, some of which have been touched on already in the thread.
But one of the biggest factors is Tumblr. Tumblr, for the uninitiated, is a microblogging platform that became a huge fandom outlet in the 2010s. People weren't writing fic directly on the platform, but there were a lot of visual edits. Playlists, moodboards, that sort of thing. And a lot of fandom discussion. Some of it was serious analysis, some just fun memes. One of the USPs of Tumblr was that it was an anonymous social media platform. People interacted with you, and built a persona around you, based on the things you chose to share. So you might not know someone's real name or face, but you'd know they made that piece of art you liked and posted about their crippling anxiety once, and you really related to that post. Creators gained followers not through looks, but through personality and fandom clout. Remember this. It will come into play later.
So it's the early 2010s, and gay rights are being pushed to the forefront of pop culture, as the civil rights fight of our time. Lady Gaga just put out Born This Way. Glee is inescapable. America has just legalised gay marriage. Modern Family is getting big. Ellen and Portia are beloved. Ellen Page has come out in a speech that was universally praised for its bravery. Macklemore has released Same Love, and taught the world the power of straight allyship. Gay is in.
Tumblr is a very progressive platform, so naturally, people start loudly declaring what great allies they are. Homosexuality is capturing the imagination of middle class American girls. It feels like the last true struggle for relationships, in the modern age. Girls really romanticise the idea of all that pain and suffering. The idea that society judges you and your family might disown you, but your love is so great you risk it anyway? What could be more romantic? The concept exerts a strong pull on fandom imagination, and it's around this time you see an explosion of gay ships in fanworks.
But of course, the best fanfics are written by gay people. They're the most authentic. Jealousies arise. Discourse begins, about whether or not it's appropriation for a straight person to write gay fanfiction. This all leads exactly where you might expect: to a sudden explosion of "queerness" in fandom circles. Suddenly, every creator is bisexual! Pansexual! Asexual! Demisexual! As long as you have a label that isn't straight, you're good to go.
Then around 2015, trans mania hit and being gay became yesterday's news. Being straight was still passé and the ultimate sin, but now that everyone was identifying as some flavour of the rainbow, being gay didn't get the attention it once did. So girls moved on to identifying as non-binary. A hardcore few, who had really swallowed the propaganda, followed this all the way through to transition. There's a breadcrumb trail that essentially leads from Born This Way to our friend Lars, and it leads through Tumblr. It is my honest opinion that you cannot separate the trans contagion of the 2010s from Tumblr. Tumblr was the crucible in which it was all forged.
I think a lot of people have just become stuck in the Tumblr cultural zeitgeist, and are writing slash fanfiction when they'd secretly much rather be writing hetero fanfiction. But they're stuck on the idea that being straight is boring and a little bit morally wrong, actually, and the way to get really big in fandom is to go gay. I do think we're reaching an oversaturation point though. The pendulum is probably due a swing back.
Slash fanfiction will always be popular with women - but you're right, there used to be more heterosexual pairings in the space. The cultural dominance of TQ in fandom is why the balance is out of wack today. But it'll shift. The TQ has been in ascendence too long. It's becoming something kids cringe at. In a few years I think the landscape will look more like it did in the past.