Like the topic says, why so many? I first noticed this when checking the top iCame 100, then I realized that a (from my POV at least) disproportionately large amount of porn involves characters from said shows.
The iCame list was rendered obsolete by bot accounts making troll votes. It doesn't mean anything now.
And these cartoons have a lot of R34 because just about every single person in the west has seen the animated sitcoms they're from on television. They still have substantially less R34 than characters that are actually appealing from a visual perspective with similar levels of popularity; they're just overrepresented because of how many people know about their series.