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.
Well there 2 types of criminal activities online and in live one which in online is they used bitcoin or crypto as a payment since it is easy than fiat but in other type they used the physical one which it is easy but criminals now are really careful they are using the internet and crypto to not be easily detected