Iowa State was added to the B12 because the B12 and the networks wanted to lock down the huge and coveted Cedar Rapids- Des Moines TV market for the B12.
Otherwise, it must have been for Iowa States historically great football teams over the last 3 decades. Or maybe the blindfolded B12 Presidents threw darts with 20 school's names on a dartboard, and Iowa State was hit by the required number of darts for admittance. Otherwise, who knows what the perceived " value " was for Iowa State football for the B12 when their lucky number came up in the College Football Lottery of Life of the Haves and the Have Nots. Its over my head, I'll admit.