Criminals prefer fiat because they find it faster and easier without accessing anything on the internet. They see it as less complex that they could do transactions even without the use of the internet. However, some modern criminals prefer using cryptocurrency because of the anonymity that it provides. Traditional criminals aren't knowledgeable about cryptocurrency yet.
like you said because they really prefer things that are instant and don't want to be complicated about complicated procedures.
maybe even compared to fiat they prefer real money directly when transacting because indeed this does not require excessive things and is very simple to do.
As for what you said, there is some truth to anonymity and until now bitcoin is still considered a tool for terrorist financing based on this.
but actually if we look at the current cases it is very rare for this to happen
I think there are two ways to look at it.
1) When it comes to hacking or siphoning off money, I expect it to go the same way as a bank robbery. Before the age of internet, bank robbery implied a group of men physically entering the bank and robbing money/gold. Did it change as soon as the internet began? No. It took some years for criminals to understand internet and how to use it effectively to siphon money, which is very ubiquitous in today's world (hardly we get to hear about physical bank robbery today). With the pseudonynomity of bitcoin, it becomes relatively easier as compared to creating a fake account and closing it as soon as the transaction is done.
2) However, for money laundering, the two important differences are: a) It's mostly done by people who are well known in public space such as politicians, heads of big organizations, celebrities etc. as opposed to hacking which is mostly done by unknown people/groups and b) Even with so much advancement in the notorious use of internet, money laundering is largely restricted to cash. This means that money laundering people care a lot about anonymity. Although people claim bitcoin is anonymous, it's pseudonymous to be honest. And history (Mt. Gox case for example) shows us that stolen bitcoins can be recovered. I'm not saying that people won't use bitcoin for money laundering at all. They will and may be that number might increase with time due to better understanding of crypto, but majority of them will stick to cash manly because of the anonimity and difficulty of tracing cash transactions.
Let me know what you guys think