1. Handle in large quantities. The ransom paid here was $1 million. With crypto, transferring that much is a few keystrokes. With cash, it's basically a large bag of cash that needs to be moved around.
2. Hold for ransom. Given point #1, kidnappers (of people and data) know that it's much easier to demand payment of large amounts in crypto than cash. If you kidnap a rich person in the crypto world, kidnappers know it's easy for him to sign over a very large amount of bitcoin quickly, which looks to be what happened here. Even for a very wealthy person, it's not logistically easy to get a million dollars in cash, never mind hand it over in a way that isn't a huge risk for the kidnappers.
3. Launder. Crypto tumblers are still a thing despite US government crackdown. Laundering cash with recorded serial numbers is much harder.
I'm not denying that crypto may have some legitimate uses, but it's basically a dream come true for criminals.