I agree that some people have reasons for keeping blockchain transactions private. But criminals have much stronger reasons to keep them private.

The most useful cryptocurrency is not as broadly useful as other means of moving money around. So if it has advantages for criminals, criminals will use it disproportionately. Ransomware generally doesn't take Visa and Mastercard. The Silk Road was Bitcoin's most prominent early adopter. And the early Bitcoin market was also rife with scams, fraud, Ponzi schemes, and all manner of other criminal idiocy: https://www.amazon.com/Attack-50-Foot-Blockchain-Contracts-e...