For Tenacity to be good, it would require another stat to not be so good. If DET were to change to a DPS only stat, with Tenacity getting its benefits for tanks, you'd see a lot more Tenacity. This would also drive down the amount of variance though, between Crit, Tenacity, and Skill Speed. Though there will always be 2 stats to rule them all in this game, nothing short of a complete change in how all substats work would change that.