One would expect the ERC-20 USDT version to be popular. That certainly has been the case in the past. Since the initial issuance in early 2018, the amount of USDT on Ethereum has increased gradually. At the peak in late September 2019, over 1.022 billion ERC-20 USDT is in exchange wallets. A remarkable amount, albeit it has never been reached since.
maybe it is because investors are not confident enough to invest in such market where it can not be predicted what will happen next. investors are holding ustds in their wallet and waiting to buy low.