I think USDT will be more meaningful when the condition of Bitcoin is deteriorating, because crypto users prefer to hold USDT. But when Bitcoin has started to increase, then this stable coin will be abandoned, and switch to Bitcoin. I think it's a risk of a stable coin. When crypto users switch to Bitcoin, that's when the trading volume will decline.
This is probably the reason if the USDT supply decrease tallies with the increase in price of Bitcoin and other cryptocurrencies.
Another possible reason is the availability of more useful and easy to use stablecoins. I guess USDT should develop more useful features to attract more users.