Because DPS is pretty free most of the time. For 90% of the content in the game, it's possible for DPS players to get by without really having to do too much. While some end game content might require DPS players to perform at a certain level (and for what it's worth some of that end game content can ask quite a lot of DPS) and present some difficult mechanics for them, most of the playerbase probably won't ever even deal with that. And a lot of those that aren't up to snuff and end up getting carried (one way or another) will end up spending a good portion of those fights with their faces on the floor. For most people, DPS are going on a ride, while tanks and healers are but facilitators for that ride.

Tanking, on the other hand, tends to have a much more consistently higher set of standards for players to meet. In addition to being made to pay attention to a much wider array of information, tanking players typically also need to have an understanding of how every encounter works, instead of getting by by just spamming a rotation. Furthermore, in the case of DRK, in addition to having its own host of issues as a tank that need to be addressed, it presents the most complex gameplay of any tank while also having a massive skill bloat. So by having a bunch of players jumping into a role that demands more of them with a class that is the hardest of the bunch to approach this new role with, it's not at all surprising that most are going to give up very quickly. And it doesn't matter how cool you make DRK (soooooo cool), you're never going to make a tank that players who don't like tanking will enjoy.