Cryptocurrencies are basically securities. If it's difficult or impossible to exchange fiat currency for cryptocurrency and vice-versa, the value of cryptocurrency drops to basically nothing. What's the point of owning a security you can't sell to pay your taxes/mortgage/electric bill?

Even if cryptocurrency<->fiat transactions continue to be legal in other jurisdictions, making it illegal to trade USD for $crypto would make it very hard for a US company to pay cryptocurrency ransoms making such schemes much less lucrative.