Good question. I'd say either the vast majority of tethers are held by the people behind Tether, and/or the non-Tether people who hold it are massively stupid. Any non-Tether people holding Tether are running a risk, with literally no long term upside. They say cash is the worst long term investment, well, Tether "cash" is even worse.
Nominally I'd think it's because they can, theoretically, be exchanged for $1 USD. Practically that's not possible currently (by the sounds of it), but it's the same reason people don't go around selling $1 USD bills for $0.90 USD.