Meanwhile, USDT in terms of audit, regulation and underlying storage is less transparent. so many sell it and switch to USDC and BUSD. maybe this is also one of the reasons. Moreover, there have been problems in the past that have tarnished the reputation of USDT itself. although USDT at that time was able to solve the problem well. But people are still afraid and feel that USDT is still a high risk.
Secondly, USDC didn't grow all that much and neither did BUSD but that could be mainly because of the market going down as well, people might be leaving all together instead of just turning their crypto to stablecoins. This UST deal was just something major and hurt all of the market and not just even stablecoins.