Why isn't Do Not Track enabled in Tor Browser Bundle by default? I'm aware that it's ultimately up to each site to decide if they'll honour the request, but wouldn't it be sensible to go ahead and enable it for the sites that do?
This should not be considered an authoritative answer, but my best guess is:
So, in short, it's unnecessary to enable this, and doing so may actually reduce your privacy.
Additionally to what Michael Hampton said the maintainer of the TorBrowser is not in favor of DoNotTrack (DNT), because it is just another bit to fingerprint on and might not be honored at all as it is privacy by policy, where privacy by design approaches really work.
For me personally it is also a short-coming of DNT, that it is implemented as black, white or gray. I couldn't allow a website to track me if I want to block some others, or the other way around.