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.
I agree. Low profile criminals don't care about decentralization maybe because they find it hard to deal with Bitcoin and find it easy to use the usual payment system which is fiat. High profile criminals with high volume transactions find themselves in love with cryptocurrency to evade law and of course authority that keeps on tracking them. Humans keeps on finding ways to survive and that is the way it is.