The appeal is their personalities and character development. The show has been going for over 20 years, so the boys have been fleshed out a lot, which makes them more endearing.>>3498436
Because it's weird, to put it simply. They love absurdist humor, and a lot of the satire in SP references things that they don't know about or understand, which makes it all seem even more random and ridiculous. It's like Pop Team Epic to them, but with cute shotas.