This is just speculation, but it's something to think about.....
Maybe four weeks ago, Trey and Matt were *really* planning on showing the episode that tells who Cartman's father is.
But then, notice how after "Cliffhanger" aired, South Park became super-popular-It was on magazine covers, more T-shirts and other stuff came out, raido stations talked endlessly about SP. Right now, South Park is the cool thing. Not that theirs anything wrong with that, but it seemed to me that if this kept up, SOuth Park would become too mainstream. And I don't think that's what Trey and Matt wanted.
So maybe they decided a few weeks ago to show this T & P episode instead of the Cartman's father episode to ward off any SOuth Park "bandwagon jumpers" away from the show in order to keep South Park more "underground". I think it's a good idea. Obviously, after last nights episode, we found out who the true fans are.
Missy