The structure of risk is similar for both Tether and Circle, AFAICT. While USD risk is heavily distributed across many regulated banks and there are measures in place to mitigate bankruptcy, USDC/T is fairly centralized and the fall of a single bank can put the entire currency at risk, cause sudden and extreme inflation (as is the case now), and potentially stop being accepted by even more merchants.

USD risk exists, but seems lower than USDC/T risk, and the same is true for its volatility. All in all, I am not surprised that there are more merchants that accept USD than USDC/T, and the change in methodology that would make the latter competitive would likely require taking a page from the Federal Reserve, not vice-versa.