Fiat money is easy to get and more reliable to accessible. You can easily steal fiat than cryptocurrencies.
Blockchain innovation makes their security more intense to break and criminals would prefer not to waste their time by hacking it.
But if the criminals are smart like a scammer ( who may consider as a criminal ) it can easily misdirect people and stealing their coins.
This might be true in the last few years. But now with Bitcoins being above 50,000 USD it becomes very rewarding to steal just one 1 BTC. I think the risk/reward is what drives criminals into the crypto market. If you get more money for your work then it becomes more attractive to go for coins at the moment. I would expect for criminals to try and convert any stolen coins to FIAT currency very quickly and just store their wealth in cash somewhere easily accessible.