Subject: Several reasons, honestly
Posted on: 2021-06-28 17:09:10 UTC
Firstly, OTW (the Organization for Transformative Works, the not-for-profit that runs AO3, among other things) has attention, and thus donors. OTW also gets attention and support from other nonprofits. Because OTW is nonprofit, AO3 is not obligated to make money, and because they're run by donation they are not beholden to advertisers. AO3 was designed with the express intent of being an archive, and the content rules are lax. There's not a lot that can't be posted to AO3, and very little that is grounds for removal. The founders have spoken on this and they basically said this was a deliberate choice. AO3 is intended to preserve fan content regardless of its type or merit. And of course AO3 is run by volunteers, and everyone involved is pretty passionate about keeping it as a refuge and archive for fanwork.
So basically so long as the OTW doesn't suffer a hostile takeover and they have enough money to keep the lights on, there's not a lot that can go wrong. So, y'know. Donate if you can.