[60secz]

Because the value of cryptocurrency is strongly effected by the volume of buy and sell pressure. Market cap implies that a level of liquidity which is likely not present -- it can very difficult to sell large amounts of most crypto without at least temporarily tanking the value.

[eloff]

Market cap doesn't imply liquidity, not for equities either.

A larger market cap might be correlated with more liquidity, but you readily want to look at shares outstanding and average volume to measure liquidity. I don't see why crypto is special here.