If I were to receive a ransom payment, I'd much rather do it with cash rather than cryptocurrencies. Using a technology whose main point is to have a immutable ledger of all transactions, forever, seems to be against my interest of wanting to be able to hide the receive ransom transaction somehow.
I think the key is that if you have to pick up money it requires someone to be physically in some location. Where they can be arrested. The rest of the ransomware experience from the attacker side is hanging out in their living room in their non-extradition county. Why ruin a good thing by going somewhere were someone might have a gun.
[Of course an immutable ledger might allow people to use a gun on you in the future, but it pushes the experience into an uncertain and hazy future. Safe to ignore in the present.]
