There's a non zero chance that your criminal extortion scheme won't end up with bill serial numbers tracked by federal authorities. Hence that's probably why 97% of ransomware uses BTC.

That's a massively higher share for cryptocurrencies than in legitimate transactions, isn't it?

That's awkward, given that:

* "All the bitcoins in the world [are] worth roughly $160.4 billion" - Investopedia

* USD M2 (2018): $16.1T - https://tradingeconomics.com/united-states/money-supply-m2

So really, BTC is used at least as much, relatively speaking, for crime, as USD.