The whole expansion thing used to be about football--that's why we took Nebraska.
I don't think it is anymore. We didn't take Rutgers and Maryland for their football programs, or their great tradition.
Maybe the calculation was made that the Big Ten is a strong-enough football conference. Maybe the current members said, "No more great football programs."
Maybe the BTN needs higher ratings in December, January, and February.
Maybe the next picks will also be basketball schools: KU and UNC would fit.