People usually don't like to play tanks.They don't feel "usefull" i think.They think they don't have the feeling to "carry" their team.
Having a main char with only dps on it(and a tank leveling atm)and another char War-PLD and WHM,i can tell i REALLY like to tank but ONLY with the war.I can't stand with the PLD it's just boring as hell.But i do feel myself really useful.I feel "powerful" all time when playing tanks.
I just think people want the "easiest" way.There is nothing hard or easy in this game honestly it's all about teamplay.But well i still don't understand why there is low tanks in the game(and MMO in general actually)