The problem is that DNT is not a technical measure like pop-up blocking, where enabling it by default is guaranteed to have an effect. Instead, it's a voluntary measure whose implementation depends mainly on the advertising networks rather than the browser vendors.

So if we want DNT to have any effect, it must be something tracking companies will consider a meaningful signal of user preference. The risk in enabling it by default (for a browser that will eventually be used by 20-30% of the web) is that tracking companies would increasingly ignore it -- even the ones who have pledged support for DNT in its current form as implemented by Firefox/Opera/Safari.