Liquidating the privacy coins into fiat cash is one of the biggest barrier to them because they can't stay completely anonymous while using internet at some point they will leave their traces which could be never erasable.And also not much of those criminals doesn't know the existence of cryptos that is why they prefer hard cash over banking transactions.
I agree with you. Some criminals preferred cash or physical money rather than crypto currency because I believe that some of them have no knowledge in crypto world and they might get caught with just a wrong move that they will make. This criminals just wanted an easy to use money that is why they tend to always choose fiat for a sure spendable money, anytime and anywhere.