Since decentralized crypto can't be controlled by the government, it's much harder for them to monitor it than the cash that they're the one providing the community. And with crypto transactions it's also harder to track than paper money. And it won't be that suspicious if you transfer big amount of money with crypto unlike cash.
They don't have to control crypto. They have to properly regulate the businesses that handle crypto.
Of course, it would not be a perfect measure, but money control is also far from perfect.
Being strict on crypto - especially on individuals - will most probably solve nothing and also may restrict human rights. Plus, it's not an easy task, especially as crypto is not physical.