I think it has to do more with the fact that 5.1 is on VHF. They broadcast a little ways north of Des Moines (biggest city in Iowa) and a lot of people have UHF-only HDTV antennas. I think they do a low-power UHF broadcast on 50.1 to get Des Moines covered. Since I live in a suburb, I don’t think the signal is strong enough, but I do have a VHF/UHF antenna so I can get the VHF signal where I am.
My Simple.TV lists the channel as WOI-LD, not sure why that is, it should be WOI-DT. Perhaps LD is the lower power output channel?