Coincidentally, America and Blake have very similar pedigrees. Both were in The Sisterhood of the Traveling Pants, both star in popular TV shows (in America's case, it's Ugly Betty, a tongue-in-cheek look at the craziness that is the fashion world), and both are super gorgeous, young and famous. It should also be mentioned that Gossip Girl averages about 2.5 - 3 million viewers a week, whereas Ugly Betty averages between 8 and 11 million. In case you're wondering what this means, it means more people watch Ugly Betty and ergo, it's the more popular show.
So, why on God's green earth did La Wintour choose to put Blake on the cover and not America? Hmmm, could it be that it's because America is......gasp! Latina? While, Blake is as white and all-American as apple pie. I don't know, but it was a thought that crossed my mind ;)