I would think being the two largest cities in bordering states with significant business / economical / cultural ties would be obvious enough why there MIGHT be a large amount of seats between the two said locations, but maybe not to everyone.
Never lived in CHI, but I always felt that the business ties were stronger from CHI to places like MCI, STL and IND as far as heartland markets go. Just because MKE is less than 90 miles away from CHI doesn't mean they have the same demand to the same places.