They are better regulated and therefore less risky than Tether, which raises the interesting question, Why is USDC circulation only 20% of Tethers...
The most likely threat to Tether as a currency (IMO), is that one or more large countries outlaws its use, and/or decides to prosecute its owners...
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
You just answered your own question.