ExtraPenguin

I think it's that for mainstream stuff, the readers might have read 10 such things before breakfast already, so the only way to "win" a comment is to be memorable/unique. With rarer stuff, you might be the only person to ever write it for the fandom. This means that rarepair shippers etc will have all those years of squee they can only rarely indulge, and thus the comments get longer and more involved, because they want to encourage you to write more. A similar principle applies to more unique mainstream stuff – it's the only mainstream popular ship/character/setting that has dragons/politics/a realistic look at the inside of a nuclear power plant/whatever, so they're there via the mainstream thing, but squeeing about the unique concept.