Some bots require to use USDT. Particularly if their deal start condition uses a paid or TTP signal. Also if the bot requires any of the 3Commas built-in deal start conditions like: RSI7, TA presets, TV signals (strong buy/sell, etc), and many more.
Bots using USDC can be interchanged to use USDT if you don’t suffer from FUD on USDT (there’s been fears about a potential USDT delisting for years).
The reason TTP uses USDC is so we are not holding only a large bag of USDT.