Privacy

Nah, the idea was sound. When Do Not Track was introduced, most jurisdictions had privacy laws which required users to opt-out. Sending this DNT header could have been an indication of users not wanting to be tracked and therefore would have served as legally binding opt-out.

It was Microsoft that killed it, by having Internet Explorer send the DNT header by default. When it's sent by default, without users actively choosing to activate it, then it cannot serve as a legally binding opt-out anymore.