Because teams like USC and Texas would rank low and wouldn't be anywhere near Alabama, Georgia, and Ohio State.  Looking back just one year, Oklahoma would also look bad.  However, looking back many years, USC and Texas look good.

I'm assuming that you really mean on-field success as an estimate of future success, with the assumption that past history is a good indication of future performance (hey, it works for stocks!).

One other thing, for realignment, recent historical on-field success is likely one of the least important factors.  USC, UCLA, Rutgers, Maryland, Nebraska.  Based on recent on-field success, none of those teams should have been even considered for an invite to the B10.  The other factors are far more important (including expectations about future success).