That's because the device is input only. By default Ardour uses the same device for capture and monitoring. The reason is that different audio devices have their own internal clocks, and using more than one devices introduces clock drift.

You get around this be setting JACK to use the snowball device for capture only (and by doing so you are using its clock) and then attach whatever device you are using for monitoring as a JACK client using zita_j2a from the zita-ajbridge tools package.