Everything can be used for illegal transactions. Probably USD in the form of cash is still the most used tool for illegal activities. I guess EUR is the second.
I am talking about it here for a long time. I really like that smbd thining like me

Criminals don't care what to use bitcoin or dollars only profit make sens.
I think he already calculating profits they need confidentiality, security and anonymous they would prefer cash rather than a bank or use online. That's all I know