Because of the "nor shall be compelled in any criminal case to be a witness against himself" part? How does that stop the IRS from telling law enforcement that you've suddenly come into millions of unexplained dollars?
You already have to be in trouble. The cops can have the IRS release your tax returns, the IRS doesn't call the cops(they have their own).
>IRC 6103(i)(1) provides that, pursuant to court order, return information may be shared with law enforcement agencies for investigation and prosecution of non-tax criminal laws.