Just about every Ethereum competitor has some equivalent to ERC20 tokens. I'm surprised Tether dedicated 1 Billion USDT to TRON but its likely because they don't think there will be any repercussions for creating liquidity out of thin air.
So all arbitrageurs, market makers, etc, transfer USDT between different entities through TRC20 network. USDT on TRC20 started to increase only after Ethereum gas fees got ridiculously high and it became too expensive to use even for pretty high volume market makers.
Also, market makers moving Tether around do not give two shits about $60 in gas fees or whatever. Additionally, a huge amount of Tether has gone into DeFi on Ethereum and L2s (or bridged to newer L1s, which typically don't have a Tron bridge).
Ethereum was dominating TRON until very recently and this does have to do with huge increases in Gas prices on Ethereum