1) Able to be used on both Beasts and Humanoids, it is an extremely effective crowd control spell, used everywhere from Wailing Caverns to Serpentshrine Cavern to Sunken Temple to Black Temple. So many mobs are susceptible to Polymorph, it is almost a universally good crowd control device.
2) It is spammable. Some crowd control abilities, like a hunter’s freezing trap, come with cooldowns attached. Polymorph does not have a cooldown.
3) Polymorph is a crowd control spell that is brought to the mob in question. Again comparing to a hunter’s freezing trap, polymorph has a 30 yard range, and as such can be casted at a mob that is still yards away. Freezing trap requires the mob to come to the trap, which requires a lot more precision and skill to use properly. Any dumbass can use Polymorph correctly.
4) The mob barely moves once controlled. A warlock’s fear “controls” the mob, but it is still running about like an idiot. There is serious danger that other mobs will be pulled. A sheep doesn’t leave the same 4 yard zone, and most importantly, if other mobs path over it, they do not aggro. Apparently, there is nothing suspicious about a cute piggy named “Incredibly Powerful Naga”.
5) It lasts a very long time. Max ranked Polymorph lasts a hefty 50 seconds, which means in your average instance, you can sheep it, and then forget about it for almost a minute.