Exactly St. Louis probably doesn't have a lot of KU fans, or maybe not that many college basketball fans, so there are that many more tickets for KU fans in KC and other parts of Missouri. Plus St. Louis already has an advantage because there are more tickets to sell.
I'm sure there will plenty of tickets for KU fans, or whoever, no matter where they go, but I think the Des Moines site is going to require them going to secondary tickets markets because there are less to start with, and since it's Des Moines's first year, many locals will buy tickets immediately.
Are you trying to go out of the way to be that paranoid? It's simple - if KU is one of the best teams in 2016, they will be playing in Des Moines. The whole "Well, St. Louis doesn't have that many fans so we'll make KU fans go further out of their way to play in St. Louis" is just you and your "The NCAA will go to any length to screw ISU" mentality.
If KU is one of the best Midwest teams, they'll be playing in Des Moines. /discussion