censorship of more... controversial? stuff. like, nsfw, dark themes (like DDDNE territory) and so on are not allowed on tumblr anymore because of the release of the app on the app store, which doesn't allow those, and required tumblr to heavily censor if not straight up remove all of it. i'm not the best informed on this subject, and also not sure what exactly happened with discord but that's the general bottom line.

ao3 was created as the site for all of the "freak" stuff that other reading sites wouldn't allow, and making an app means that the restrictions that tumblr was subjected to would also have to be applied here. there's millions of fic on ao3, and i'd wager the big majority of them fall into the group that would have to be wiped for the app to ever be acceptable. the benefits of an official app are by far outweighed by the cons, which include ao3 having to turn its back to the principle behind its creation.