Not that I'm very informed about this season, but I suspect Fox didn't want to splash out on more than 6 episodes. I mean, if there's someone in the production line who wouldn't care about the 10-13 thing, it would be them.
This is at least the 2nd time the show missed a number-related opportunity. The other was of course when there wasn't a movie on 22nd December '12. I'm still not over that one.