My guess would be that the USB3 port doesn't suppport video.  Not all of them do.  I had the same issue one time with a USB-C port and read in the manual that no all of them have a video processor tied to it.  I don't believe that it's automatic with the USB3 spec to have video support, but it has to be tied to a video processor.  It would depend on whether the motherboard manufacturer designed it that way.

That would be really lame but thanks for the heads-up. 

Yeah, I was very disappointed with the USB-C controller on one laptop I had from work that didn't support the second monitor.

It could also be that it's just that the motherboad has a single graphics processor.  To test that, remove the regular HDMI and try running JUST the USB-3 -> HDMI display.  If it runs, it's just because there's only a single GPU on the board.

Another scenario I've seen is where, yes, you can run two displays, but you can only mirror the two screens (make them show identical content), but not actually extend them into two different monitor spaces.

Do you have a manufacturer / model of your PC / laptop?  Mine was an HP, and if I recall I ended up calling their tech support, and they told me that.