I have played World of Warcraft since vanilla (I started in 2005), but the writers never explained why you as a player (regardless of race) can be resurrected, yet heroes like Cairne Bloodhoof, Varian Wryn, or Voljin are cannot be.
Is there a reason for this that I have missed?
It's just an in-game mechanic, that has 0 effect on the story.
If death wasn't permanent at all then the story would become boring and pointless ... would death be permanent every time a player mistakes the height of the jump he takes the game would be impossible to play.
The only lore connection it did have was that Spirit Healers were Val'kyr that split off to bring noble souls back because their destiny wasn't completed yet... But this has been retconned two times already and is no longer relevant.
There are a few exceptions where it is tied to the lore but rather irrelevant like in The Scarlet Enclave you play a death Knight, an undead servant of the Lich king. He doesn't allow you to die so he sends his Val'Kyr to bring you back as his slave over and over until your task is complete.
I'm not sure about the other classes, but there is an in-game reason given for demon hunters to come back from death. The first time such a character dies, Illidan says:
Interesting. Like me, you have an immortal demon soul. You cannot truly die. Instead your soul will make its way to the Twisting Nether. There, you will wait until you can find a suitable body to inhabit.
