In 807, Blanket tells Kyle that he has a half brother and a half sister. What he doesn't know is that he has another half brother: Kevin! And Kevin's genetically engineered origins are similar to Blanket's. I wonder if they considered bringing Kevin into the episode.
But here's a crazy theory of sorts: Michael Jackson paid them like a million dollars to make the episode. I mean, come on, he was indicted the day after it aired. (The South Park curse strikes again!) Plus, a number of people have mentioned in this newsgroup that they didn't give Jacko the full treatment. Instead, they reinforced the image of him as an eccentric but innocent Peter Pan.