Some USB cables are not designed to carry data ... just power. Unfortunately there isn't an indication on the cable itself.
For the UT800 to show it needs to be in charging mode and you need to use an ANT stick for Garmin Express to see it.
What was the reasoning behind needing an ANT stick to communicate with the UT800?
I don't know is the short answer. I could make some guesses. I don't think there have been any software updates for it since it was released so it should not really be an issue.