I agree, and it does surprise me, but I think it's a mix of 1. tether has been around a lot longer and is on a lot more exchanges 2. tether is.. less well controlled by US regulators and hence better for.. questionable exchanges to list.
Of course it is possible that Tether are getting custom, but not the kind that wants to leave a bank trail. But if that's the case they're going to fall foul of the regulators sooner or later...
Market makers and trading shops with 7-8 figure funds playing with riskier cryptos or trying to tap worldwide volume.
It's not MicroStrategy and Mass Mutual Insurance using Tethers