There's no doubt in my mind that the reason for this is hugely related to Ethereum's high transaction fees. Although I never used USDT because I prefer BUSD, I would also pick the TRC-20 version if I have no other options. Let's be practical.
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Indeed, it's because of the high transection fee on ethereum blockchain. Also, Tether already minted so many TRC20 USDT which is so popular to make transection fast and pay less than other blockchain. But ever exchange incrased the withdrawal fee after this current hype of using TRC20 usdt instead of ERC20 usdt.