This issue will never be a lack of interesting tanks to appeal to people. The issue is the responsibility of tanking that a majority of people would rather not deal with. It's much easier to do what you're told and do damage as a fun class than it is to control the pace of a dungeon, marking targets, tank swapping, kiting and any other responsibility a tank might hold. You will find this trend in every MMO, not just this one. Fun is subjective but a majority of players are DPS for these reasons. Adding a new tanking class inflates the amount initially but it's always trended back to the issue of DPS being a dime a dozen.