Maybe criminals prefer Fiat because of its high level of adoption in the mainstream world? Or maybe because Fiat has an easier learning curve than crypto?
Criminals are obviously various types, from idiots to smartest one. Now the thing is first category might be not smart enough to understand the cyrpto based transactions and perhaps they are man of traditions! If you buy-sell things with the fiat then it is pretty fast and on the spot. I don't think anyone would stand on street for hours to get their payment which is being received for the crime that they just did.
If by any means criminals start adopting crypto in mass (especially privacy coins), we could expect a full government crackdown on the entire industry. Which is why I think it's best for criminals to continue using Fiat for the sake of the crypto/Blockchain industry's growth worldwide. But we'll never know what will happen in the future. Thoughts? 
If smart group is using this one then they know the value of anonymous transaction.
However, there is loop hole in such transactions. They can not easily launder huge amount of money which is coming from the crypto space. Today or tomorrow they will need the crypto's converted to fiat for their daily needs. The only source they can the money is via their bank accounts.
Just imagine millions of transactions getting recorded in the bank account will surely sound the alarm for local authorities.
So it's still confusing for them and good for us since we will get great deal of volume in the crypto space.