I'm confused...did Ohio State join the Big 12, or did Boise State?
Anyway, Ohio State brings a whole lot more fans to bowl games. Boise is more deserving, but OSU is only a spot below.
Automatic bids went to the 6 major conference winners, Texas at #3, Alabama at #4, and Utah as the highest ranked non-BCS school and at a high enough ranking (I forget the exact criteria, but part of it was to be ranked higher than a BCS champion). That leaves one spot, and the eligible teams (top 14) were Ohio St, Boise, TCU, and Georgia Tech. TTech and Okie State weren't eligible since the Big 12 already had two spots. There's no requirement at all to take the next highest ranked team.
It also wouldn't surprise me if there is a bit of pressure on the BCS bowls to keep the big payoffs within the major conferences.