Reddit could have simply banned T_D at any time. In the years since then they've definitely started banning subreddits for no good reason, but apparently based simply on how much upper management likes the subreddit. Presumably, T_D was not banned because upper management liked it.
On the other hand, there aren't simple technological fixes to social problems. T_D's mods remained tricksy and continued to work their userbase to upvote and focus - in ways which didn't breach the sitewide rules on manipulation - and still kept hitting /r/all