At one point in time there were a dozen trikes in my club. There still may be, but only one guy ever rides his with the roadies, and at one time he was just shy of being a pro. That old line about Lance being able to keep up even if he was on a Huffy? Yeah, that's this guy; and the trike is almost too much work for even him.
I think OSS was so much more popular than USS in the States that the domestics just stopped making USS except for trikes. USS became a niche within a niche. It's a little easier to find on Euro recumbents. I just find OSS more practical. There's places to hang computers and bells, it's easier to walk the bike, and there's still no pressure on the hands with OSS.