I don't know this as fact, but I would assume that OSU and UM most likely have the largest and most lucrative (traveling, merchandise, etc) fan bases in the Big 10.  Therefore, from a Big 10 financial perspective its not necessarily about getting a rematch of those two teams in the championship game (which they would love and make a killing on), but at least having one of them.  In opposite divisions, they aren't mutually exclusive and thus have a better chance of at least one making it (don't have the time to crunch the actual stats on that, but would be interesting to know what the actual change in probability of at least of UM/OSU making it is).  Basically, it decreases the probability of a dud championship game.

I'm completely against the change mind you.  Money be damned.