There's a good explanation of why Cartman has remained a beloved character despite being a literal bastard child.
You see where most shows like Rick & Morty, Bojack, Steven Universe, Owl House, and yes, both Hazbin Hotel and Helluva Boss try so very hard to make you sympathize with their character, either giving them tragic childhoods, live terrible lives, or been wrong by the world.
They all failed, and the characters are still terrible people who make no honest attempt to fix their lives.
So the audience gets more annoyed when they write so many sob stories for them because it ultimately just comes off as an excuse for them to be assholes.
Eric never suffered from this problem simply because Trey and Matt never tried to make Eric seem sympathetic.
They instead fully acknowledge Cartman is a terrible person, and the entire cast of South Part calls him out for his actions and for being a fat bastard.
Because of this, Trey and Matt can pull off writing stories of Eric tricking a teen into eating his parents, convince a therapist's wife to kill herself, and convince everyone to kill the Jews.
If anything, that is what makes Eric Cartman so refreshing as the supposed "sympathetic" character is that South Park doesn't make excuses for their character being assholes and thus doesn't have to waste so making sob stories.