As an option: criminals prefer cryptocurrencies less because of their volatility. Since they know that sooner or later they will be caught, they seek to get rich here and now, using the familiar and more stable fiat currency.
Most likely, criminals find it more easy to spend fiat currencies because they are accepted in all establishments unlike crypto that there are chances that they will still be traced and caught from doing online transacations. Although there is already a slow adoption for crypto that is taking place, still fiat is the best choice for criminals because they find it more easy to launder.