I would guess because Fashion Week is a bigger deal than some piddly Bravo reality series, and they don't lose much by letting the 3rd-runner-up have a show as well. It's not like they're gonna reschedule Fashion Week around PR. And I don't think the timing would work out for PR to be a fall show, which would mean starting it right before the holidays to get to this point in the season, and that's not good TV sense.