1. Criminals don't want their activities to be tracked on a public ledger. Cash is anonymous. Bitcoin is not.
2. Criminals already have money laundering systems in-place, and relationships with corrupt bankers.
3. Cash is accepted at more places. So it's more useful to have.
We're talking trillions of dollars in USD crime here. It's not even close.
That's a massively higher share for cryptocurrencies than in legitimate transactions, isn't it?
While BTC is tracked between wallets and that's easily measured, it's unclear how to measure wallets being transferred between people.
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.
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.
What payment processor would ransomware use? Credit card processors would close their account. Nobody can close your Bitcoin account. That can be used for crime, but also is a legitimate protection against the abuses of governments and banks.
Before Bitcoin, they were using stuff like Walmart giftcards.
[1] https://www.wired.com/story/was-bitcoin-created-by-this-inte...