Cash has to be physically sent somewhere else. There are already restrictions on large amounts of cash (> $10000) where large transactions must be reported to the government, and it must be declared entering or exiting the country. This is why smuggling cash out of the USA from drug proceeds is a very challenging issue for cartels.

But crypto cannot be physically inspected at international borders. So for the government, just like wire transfers, they want to know the source and recipient of such funds. If there were a service where you could bring a large amount of cash, obfuscate the origin, then wire it to a bunch of bank accounts under criminal control, that would be the definition of money laundering and also cracked down on.

In this case if Coinbase took some BTC that went through Tornado it would be equivalent to taking a wire transfer from a bank known to be laundering money.