Well, They like Terrence and Phillip, because It is violet (in ways) Immature and pretty gross....
And mostly they like it because people get hurt...

I agree with the immaturity part.

T&P's basically one never ending fart joke and to a ten year old boy(and a rather large quotient of grown men) that's the epitome of comedy - no plot, no romance, just bathroom humor.

Haha, well, they're fourth grade boys, so they still like fart humor.

Terence & Phillip Kick Ass
But When It Was The Queef Sisters
That Was Disgusting & I Almost Puked

I read in an interview,that Trey and Matt created T+P because a critic said, that their shows 'is only about fart jokes', and Trey and Matt was like...Yeah,really?So they decided to make this show, which is STILL about only fart jokes. And the children like it because...they are retarded XD okey,no,well,T+P is kinda symbol of south park.