What do you guys think is the cause of severe lack of Tank users?

[Ultimatecalibur]

So why is their APM so low by comparison even in end game content?

Because the game is not designed for double weaving ogcds and tanks need to be able to use various cooldowns in practically any part of their rotation. You can actually see why this is a problem if a tank buster or tank swap happens during a GNB's Continuation combo. Tanks actually have one of the highest ogcd action totals in the game but most of them are situational use rather than use on cooldown.