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. 

Why is this supply dwindling so rapidly, though?

https://fintoism.com/metrics/erc-20-usdt-exchange-supply-decreased-by-nearly-50-since-september-2019/