As an AAA member myself, it's probably because on other platforms there are too many anime fans for us to make an effective opposition. There are some anti-anime subreddits on Reddit, but there are possibly hundreds of anime subreddits so it's hard to do stuff there. This is probably why SurlyKong said a lot of us are transplants from Reddit (I think he's partially right, some of us have reddit accounts, though I don't). As for YouTube, some of us also have accounts on YouTube, but I haven't seen as much AAA activity, probably because there are MILLIONS of weebs there compared to us. Also we legitimately dislike anime, we're not solely a bunch of trolls like he said.
Examples include:
Boring plotlines
Unrealistic characters
Hentai
Fan service
Some anime have links to Japanese ultranationalism (to be fair, this is a small minority)
Loli (to be fair, even most weebs hate it, and rightfully so)
Past experiences with anime or anime fans that led to us developing a dislike