Couple of things
1. Anime as a whole became a lot more popular with newer generations. It also became less taboo to watch. There are so many things anime these days, branching out into games, literature etc.
2. Because most titles target a younger audience, they will have themes that resonate with teens and young adults. Humans like things they can relate to.
3. Young adults and teens are categories of people who are online the most. Especially gen z, who grew up with full access pretty much since birth.
4. Usually the logic behind choosing a pfp is picking something you like. Which for a lot of people is anime stuff.
I doubt I'd be less suicidal had I picked shows with real humans or games with non anime characters as my entertainment of choice. Hell, I don't even watch anime these days anyway lol.