Maybe convenience. Less hassle of acquiring the funds instead of converting crypto to fiat which requires crypto exchange service and mandatory KYC or by means of P2P. There's also a mandatory KYC in fiat payment processors however these things shouldn't be a problem now for criminals as it was part of the system even way back.
Or maybe because Fiat has an easier learning curve than crypto?
No doubt, one of the reasons. Somehow, people need to know the basic technicalities first of how to use bitcoin in general.
And even for let's say they are familiar, still, they found the usual payment system is more user-friendly compare dealing in a bitcoin transaction.