Because we're just not ready to let that old Chelsea joke die.
We'll admit, McCain is more desirable (ha, just reread that phrase again), but you know that, whether the Clinton campaign makes it to the White House, the speculation on Chelsea's own political career are soon to follow. So, it comes down to: Semi-decent blond with potential to be a First Daughter Meghan McCain vs. Former-possibly-future First Daughter who may one day be President herself Chelsea Clinton. (Yeah, no, we don't really see Miss Blogette running her own victorious White House campaign anytime in this millennium.)