Because a currency without a stable backing is completely volatile. Sure the value of normal currencies fluctuate, but apart from a few hyperinflation edge cases that’s at most a few percent each month.

Small cryptocurrencies fluctuate sometimes hundreds of percent each month and even the big coins can swing ±20% every few weeks. The only somewhat stable coins are the ones directly tied to real world currencies.

Having a currency that fluctuates this heavily in value creates the exact same problems as the ever changing US tarrifs did. There simply is no wax to reliably price goods and services for more than a few days. You essentially have to barter each trade.