This is the answer to your question, criminals not only use their fiat for the anonymity it brings, if that was the case then privacy coins have fiat beat on that regard, the issue is that they need to spend that money as well and how many people accept those cryptocurrencies? Very few and that will limit what they can do with their money, as such they prefer to use cash for their operations but if at some point the market of cryptocurrencies gets popular enough that you can use it to buy in any store around the world then most likely criminals will begin to use privacy coins instead of cash.