The main reason is that Microsoft enabled it by default for IE, therefore defeating the whole point of the opt-in flag. If websites didn't want to track all use

The main reason is that Microsoft enabled it by default for IE, therefore defeating the whole point of the opt-in flag. If websites didn't want to track all users they just wouldn't have added the tracking, however opting out a small number of users may make sense. Biggest downside is that the DNT header is 1 more bit of fingerprinting surface. That being said there are still some sites that support it, mostly smaller sites. For example if you set it when you visit by blog I skip loading analytics. I like analytics because it is interesting to see what posts are popular and who is referring me traffic, but I am more than happy to opt out the users who are against it.