Cash is a physical thing that needs to be taken somewhere if you're using it to pay someone off or receive a payoff. That gives means to be apprehended at some point in the chain. Crypto on the other hand you just hand over the coins. The coins are sent to an exchange's wallet, mingled, then pulled out on another anonymous wallet. This is what makes it so great for ransoms.

Not necessarily. Bills have serial numbers on them. They can also be marked.