Ninja extremely situational. Even if you hold all the advantages, you can still lose (Or flee).
They need to conform to the enemy, meaning you need to vastly overcome them.
Comparability, a pally can switch between stamina and mana, dealing damage AND maintaining survivalbility. Pallys are just overwhelmingly more powerful against more classes than ninja could ever strive to be.
This basically means pallys CONTROLS a battlefield, while ninjas merly EXPLOIT it.
(Besides, pallys are way better in group fights)