It is often easy to forget about the impact of death, permanent death, in a world filled with resurrection magic and spirit healers. For players, an untimely demise at the hands (or claws, teeth, spells, what have you) of another is merely a brief, annoying setback in the quest for xp or loot, not the final ending it would be in the real world. But unless you’re named Kael’thas, death in the world of Warcraft is still something permanent, to a point. Despite a number of retcons, there are major lore characters, with access to all the magic and spiritual prowess their allies could bring, who have suffered deaths as irreversible as any in the real world. Thus, with no real official word on the subject, it is up to us as players to determine exactly how mechanics and story interact.
Resurrection magic is available to four of the ten classes in Warcraft in a ready manner, with no cooldown or other limitation beyond mana and cast time (and combat). Dismissing this as pure game mechanics is impossible; the Scarlet Crusade (and later Onslaught) have frequent references to to resurrection in quests related to them. However, the aforementioned permanent deaths in-canon mean that there must be some sort of limitation on the use of revives, else there would be no such permanent losses. Though clearly open to interpretation, my personal take is this: reviving spells combine a potent healing effect with a re-anchoring of the soul to the body. Thus, at least /some/ of the deceased’s body must be intact and present, and the spell must be cast relatively soon after the soul leaves the body, before it can find its way to the Twisting Nether and whatever afterlife awaits. Without a body, there is nowhere to house the recalled soul; without a soul, the body is lifeless.
This view of resurrection has support in both mechanics and lore: in-game, you can only stay ‘attached’ to your corpse for a few minutes before the game kicks you out to the spirit healer, and you need to be close to a player’s body to cast the res in any case. In lore, meanwhile, bodies are often burned or otherwise destroyed (by voracious rats in the Forsaken’s case) to prevent such revivification. However, these still provide no compelling case for armies to leave their dead as corpses; many battles would be finished or calmed enough to resurrect before time was up, and there are certainly enough spiritualists around to do so. As such, I would offer this explanation: resurrection demands a heavy toll, either to be channeled (as most holy users might) or bargained for (as a shaman might), which relegates it to something used in extreme circumstances only. The Scarlet’s love of ressing could then be explained by the dreadlord who leads them; it’s entirely possible that his influence can supply the necessary strength to revive soldiers as a matter of course, rather than something extraordinary.
Of course, as I said, this is mostly conjecture on my part, and anything that works for you and those you RP with is fair game. The most important thing is to have an idea of the assumed limitations of such things before someone deflates a dramatic death with an offhand “Why don’t you res them?” or the like. The relative rarity of a player ICly killing off one of their characters makes it potentially an extremely powerful event, worthy of plenty of consideration to make sure it’s an event to remember.