Crypto was included in the laundering schemes.

"Part of the money Sánchez Gil amassed in recent years was laundered through the purchase of crypto-currencies and a large fleet of private hire vehicles registered in the name of one of his relatives"

So he doesn’t get arrested after €20M worth of Bitcoin is found in his Bitcoin wallet. Maybe there are better privacy preserving cryptocurrencies.

Not being tech savvy or not wanting to rely on volatile cryptocurrencies are also legitimate explanations for staying all cash. And he may very well have a few crypto wallets too.

Cash is king. Bitcoin doesn't even come close to it.

Survivorship bias. Those who weren't doing something dumb don't appear in such news stories

Cash is less traceable and easier to use for most criminal applications.

Nontrivial to use. Still need to pass it to someone to convert fiat to crypto. Bitcoin is a public ledger, would be even easier to tie to him in some ways.

Trade the entire wallet, that isn’t registered to anyone’s name? Seems easy enough to move money illicitly with crypto

So now at least two people - who I'm going to assume don't have 100% trust in each other - both have the private keys. How is this going to work?

Physical hardware key, so neither party knows it, but possession of it is the authentication.

Essentially crypto can be untraceable like cash or gold, it can be traded off the books, until you ‘launder’ it into someone’s name.