Well, one part is that the bad animation does bring up nostalgia and make the show feel more 'natural', thus increasing the audiences horror and delight when they do something perverted or hideously violent.
Another is Cartoon Network and it's associates are not made of money, and there is no need to waste dollars making a show look shiney and delicious* when it might distract from the joke.
Also, it's not fair to compare Japanese humor cartoons and American humor cartoons. The circumstances surrounding each and the audience thet are aimed towards is as differant as the nations they spawn from.
* I know the adjective makes no sense, but I like it, so there. razz