I also think people are able to track and trace crimes so it's a way of getting a proper measurement. You can't really do that with fiat.
The basis is only the reported crimes, usually these are big crimes and I'm sure there's more that are not reported yet.
Crypto is easy to monitor compared to fiat as paper money is hard to monitor, once it's out in the bank, it can easily be launder.
As long as there is a regulation by the government, they will be in control and will be able to monitor the movement of crypto.
That is gonna be possible as people will be required to register and comply with the KYC, that way, they(government) will know who owns the address.
I wish all the governments were consisting of honest people so that they wouldn't abuse the regulations. Unfortunately, in many countries this is not the case. In reality, governments and criminals are often not two opposing parties, there are criminals in the governments.
If we want to decrease the number of crimes involving crypto by regulating it in a given country, we should also consider the number of crimes involving government in that particular country.