I think Tucson had to be about the biggest city in the country without non-stop United service to Chicago. They used to have 2 or 3 flights a day on 737s or MDs before they cut back. Too bad it's for the winter season only, but maybe if the demand is there they may decide to keep it around.
There's a reason for it. United is simply the worst major airline in the U.S. today. When I worked for them at Tucson Airport they had 24 regional jet flights a day. Now they have 7.
There were multiple days back then when every single flight, all 24, were delayed over an hour, or cancelled. Some were delayed as much as six hours. I think almost all the high mileage frequent fliers left for other airlines.
Their reputation is in the toilet at many airports, and particularly in Tucson.