DEN is a more profitable hub than IAH and provides more unique connections than IAH for U.S. travlers. This flight is geared to people going to Italy who are based in the U.S. As a United traveler I find the lounge options in DEN much better than IAH as well unless you are in Polaris.

If SLC is getting a premium lounge on Delta than United needs to bring a Polaris lounge to Denver. Lufthansa and Turkish have many premium travlers on their flights who need access to a more premium lounge as well.