Because Microsoft made the dumb decision to turn it on by default in IE10, which instantly repelled every ad network that wanted to stay in business and destroyed any chance of it gaining a foothold.
The only reason ad networks were behind DNT in the first place was because they thought no one would use it and it would be a cheap way to divert attention away from legislation or technical solutions like adblockers. The moment there was any significant uptick in usage for any reason, there would have been a competitive motivation for the networks to abandon DNT.
On-by-default wasn't a problem because it was some kind of fundamental paradigm shift, it was a problem because it meant that the setting would actually be turned on for normal people.
If it were actually about Microsoft breaking a sacred contract or something, it would not have been difficult for ad networks to detect the IE10 agent string and apply browser-specific policies. But for the most part very few companies ever did, because if you're an ad network and you realize you can get away with ignoring privacy settings on one browser, why on earth wouldn't you do the same for every other browser?
DNT was doomed from the start specifically because it relied on advertisers voluntarily participating, and advertisers are never going to disrupt their own business model voluntarily.
I see this as a reason to blame Microsoft.
While some point companies would have tried to abandon it anyway if it got popular, there would actually be some barriers to doing that if it was already established. It also would have been easier to add legal teeth after showing the system worked. The IE thing killed it in a fragile early state so we got nothing.
To get to the point where DNT would reasonably be called a widespread standard, people would need to use it. And as soon as people started to use it, ad networks would come up with an excuse to ignore it. My feeling is that Microsoft just happened to be that excuse.
On a more optimistic note, I do think we got a few things out of DNT. We learned a valuable lesson about self-regulation -- so nowadays, Safari and Firefox aren't asking anyone's permission to block trackers and fingerprinting and they're doing it in ways that advertisers can't just decide to ignore.
DNT is valuable in the sense that it's what we can point to when advertisers complain that they don't have a seat at the table anymore.
Any reason this hasn't been up to the authors of the draft[0] to address that? Comments on a forum are likely to do nil.
[0] - https://tools.ietf.org/html/draft-mayer-do-not-track-00#page...