Its mostly laziness, stress when pulling monsters or they are the type of players who dont read all their skills. It does not help that Tanks dont need to use their cooldowns in the ARR leveling (hard mode dungeons aswell) content. When you are used to not doing something for 50 levels, this forms an habit that people have a hard time getting rid off. DPS players used to have alot of DPS buffing skills, but SE noticed how alot of DPS players did not use them, so they removed them and increased the potency of their other skills.

Whenever I see a Tank not use their cooldown, I tell them and if they keep playing lazy mode, I let them die, untill they use their cooldown. In time most learn.