Fiat money is easy to get and more reliable to accessible. You can easily steal fiat than cryptocurrencies.
You are somewhat right but I think that the reason that fiat is the preference of criminals is because they have an already established way of cleaning the money and they don't know how cryptocurrency works so they might have a problem because of Internet trail that they might leave behind.
Criminals don't have an opportunity to trade crypto with their fiat.
For what reason would they picked the crypto when they can take the cash without burning through their time.
They never do online exchanges they generally met someplace and exchange weapons and drugs.
You can not buy or exchange drugs, weapons, and illegal things online so they prefer money more than fiat.
There's another likelihood that these crooks don't know about bitcoin or blockchain innovation the less information on bitcoin constrains them to utilize fiat than some other crypto.