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.
Well, there are real world criminals and there are cyber criminals. Fiat is far more easier to use after stealing it to someone If they don't notice than Bitcoin or other cryptocurrency. So, to say the least, criminals would always prefer fiat than crypto, not unless If these criminals are modern and tech genius criminals that has a long term plan on multiplying their stolen cryptos in the coming years or If Bitcoin aren't regulated yet and still remain in the gray area that has yet to be called "legal" in a certain country.
Nevertheless, most criminals obviously always prefer cash than any other form of digital currency.