If you're kidnapping a generic very rich person, how are you expecting them to pay the ransom, a big burlap sack of cash? There's a lot that can go wrong there. A bank transfer or other conventional financial instrument? Few criminals would be comfortable with that approach. (John Grisham novels, and 'Archer's beloved bearer bonds, aside, it's virtually impossible to make this untraceable). Magic internet money is presumably far less messy.

Also, a decent proportion of crypto-millionaires came by their riches in... not entirely above-board ways (in particular, securities fraud; all those pump and dump scamcoins are paying off for _someone_), and may be reluctant to involve the authorities. And the crypto industry as a whole is unusually comfortable with extortion; hacked crypto companies paying a kind of bounty to hackers to get the rest of the funds back is a common thing.