I guess it is because of:

USDC (USD Coin) treats each blockchain implementation as a separate, native asset with:

- Separate reserves: Circle (USDC's issuer) maintains separate collateral pools for each blockchain's USDC

Tether (USDT) follows a different model:

- Single reserve pool: All USDT implementations are backed by the same collateral pool

- Token representations: Each blockchain's USDT is considered just a different "wrapper" around the same underlying asset

- Unified accounting: Tether tracks the total USDT supply across all blockchains as a single liability