Liquidity
How easily an asset can be exchanged without a big price change. Popular pairs (USDT, BTC) have high liquidity — more exchangers and better rates.
In more detail
High liquidity means an asset is easy to exchange without a big price shift: popular pairs (USDT, BTC) have many exchangers, a narrow spread and a good reserve. Illiquid coins are the opposite: few offers, a wide spread, and the exchange can get stuck. That is why rare assets are often better exchanged through liquid USDT.
