Why can't the Big 3 in each division be? Michigan Ohio State Wisconsin, and Iowa Penn State Nebraska?
One reason is that you are asking the two teams that are on the farthest edge of the conference to play each other. That is asking a lot with fan travel. If you let one group of teams just travel to contiguous schools and then make everyone else travel half way across the country, it is going to show up in team records. Particularly if PSU as the farthest team east had to play all their games in the West.
Just not a good idea.