Even if you take out all the Explict fic from FFXV on Ao3, you are still left with 18,722 fics. Which is still way more than the 2,200 FFVX fics on FF.net. The 16,000 fic difference between the two sites can't just be because one site alows porn and the other doesn't.
For a non-gaming version of it... My Hero Academia has 81,653 Fics on Ao3 and 14,700 fics on FF.net. Take out anything rated Explict and you still have 68,487 fics on Ao3. But My Hero Academia was created in 2014 and is an ongoing series.
You look at something like Naruto, which was created in 1999, the numbers are more like this: Ao3 has 49,507 fics (41,761 fics if you filter out everything Explict). FF.net has 430,000 fics. But Naruto ended in 2014.
The fact of the matter is, Ao3 was created in 2008, while FF.net has been around since 1998. So that's... ten more years of works in FF.net vs Ao3 in some fandoms. And some fandoms (like FFVII) had their original works done and over with before Ao3 really got up off the ground. Older fanfic works in the 2000s didn't have anywhere else to be published in a general fanfic collection other then FF.net. Fanfics from... 2010 and onward do.
I think it will be interesting to see what happens with the new FFVII fanfics that get written as a result of the Remake. There will probably be plenty of new writers (and other content creators) coming into the FFVII fandom and I have to wonder where they will gravitate to.