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.
Yes, Fiat money is easy to use anytime, anywhere and it is tangible unlike bitcoin which is digital currency, you can't touch and it always needs internet connection in order to make a transactions and need knowledge how to use it. Criminals don't like a complicated transaction and also if they use crypto, they need to learn it first.