So most likely,
1) they didn't launder it properly, leading to police being able to trace it to their bank accounts. I wonder if tornado.cash was used.
2) then police had their names, leading to warrants for all online accounts - google account, apple account, etc.
3) they made the big blunder of keeping their private keys in their online account. Most likely a txt file in google drive. That is such a silly blunder. Without the private keys, the police has zero proof of anything. They could have made a hundred excuses for how they got money in their bank account, as long as the police didn't have the private keys. Who keeps their private keys in an online account?
Apparently the biggest criminals make too many silly mistakes. The old saying applies here: "you don't have to be smart, just don't be an idiot"
Also, with mixers such as tornado_cash, laundering money is ,sadly, pretty trivial.
You can do that by owning crypto. No need to use it.
> Self-sovereignty
Majority people use centralized exchanges, which regularly control transactions.
> Ease of use/trade/leverage/exchange
Fiat banking is much easier to use than crypto. It's also faster. Now everyone uses 1-tap payments. Crypto transactions are more complicated than that. They also take longer. Also are bad for the environment (not as bad as media portrays, but bad nonetheless)