Bitcoin is favored by criminals because it's harder to trace back to the perpetrators when they decide to cash back into the physical world if they have used obfuscation services like a coin mixer. It's not impossible to trace but it's definitely harder than asking for a ransom in marked bills in a briefcase for example. Also the crooks can be anywhere in the world and still can commit extortion without leaving their house.
Bitcoin is perfect for use in ransoms and I'm sure it will be used/requested as ransom more often in the future but I think cash is still very good for criminals for most other things. Two drug dealers meet in secret and exchange cash for product and there's no way to know the deal ever happened without some other intel. In fact certain govs are thinking about getting rid of high denomination notes because they're frequently used by criminals to transport funds.