Cartman typically had the most and funniest jokes and almost always got his way. He exercised power over others and only extremely rarely did the way he acted have negative consequences for him. That's a pretty attractive role model for the type of kid who's going to watch South Park. Role models don't have to be consciously selected, that's kind of the point, you see success and you imitate it. South Park is an especially stupid show but you'll see this in plenty of TV - it's basically the sitcom model: some character will be an asshole to everyone around them, but the show will frame it as a joke and consequence-free. The writers want you to think that character is funny and you probably do if you continue watching, so if you want to be a funny person you probably start acting like an asshole. Most adults can't see that for what it is - kids are screwed.
And South Park was (is?) on TV during daytime hours, of course kids watched it.
The bottom line is, Cartman got the biggest laughs. He modeled that a certain type of behavior resulted in certain outcomes. Kids aren’t stupid. They see right through the overt morality (“he’s a jerk and he got his comeuppance “) to the revealed truths: being an asshole, in some contexts, can be very funny.
