It's a failure because law makers haven't made it clear ignoring it is illegal.
You probably can build a case around DNT clearly communicating that a user doesn't want to be tracked and as such it should be treated like an if the user manually opt out of all tracking.
But as long as lawmakers or court don't pin it down legally to make it a clear cut case instead of some gray area thing with a lot of wiggle room.
There is very little you can do against modern tracking tech without crippling browser functionality, as such solutions have to be law based foremost and supplemented with technology and actually painful penalties if companies try to sneak by this.
We should make it both technically hard and illegal for the surveillance industry to track us. Corporations continue to reinvent de facto government from the bottom up, and if most Americans weren't too distracted freebasing the fallacy that corporations and government are opposing forces we might be able to preserve individual liberty.
