If you look here they say that the only digital station that broadcasts on VHF is CBS-WBBM. The other stations all broadcast their digital signal via UHF. There are 9 other stations in Chicago that broadcast digital versions of their programming on UHF.
Of course, they've been slightly inaccurate in some situations.
Digital is mostly on UHF because that is where the open channels were located. There is talk that when the analog has to be turned off some of the VHF analogs will change to digital and they will give up their current Digital channel number instead.