I think this has to be one of the main reasons, don't want ISU/Iowa playing twice in a week (a split would really take away from the fun of the rivalry IMO), but don't want to risk them not meeting in the championship game. I did like it better last year when the Big 4 was the weekend before CyHawk, would be nice if they could do it that way every year. Make the Big 4 like the semifinals, and assuming ISU and Iowa won most years, the next weekend's CyHawk would be the de-facto state championship. Also the Big 4 seemed to have a little more "buzz" knowing the CyHawk was still coming up the next weekend... this weekend feels like a consolation game for both teams.
Also, I could be wrong on this, but isn't there some kind of limit to preseason tournaments? So if the Big 4 was actually a tournament, would it prevent Iowa and ISU from playing in other preseason tourneys?