I think there is useful discussion to be had as to why DNT failed. IMO it’s largely because people approached the problem from a purely tech-oriented standpoint and ignored any differing ideologies and incentives at play. An allegory to this project could be drawn.
From my perspective, a major issue with DNT was that it wasn't very clear to websites what a DNT request was asking for. The specification explicitly doesn't handle this: "This specification does not define requirements on what a recipient needs to do to comply with a user's expressed tracking preference" https://www.w3.org/TR/tracking-dnt/
