This invasion dwarfs the War of the Ancients, by Blizzard's own admission at Blizzcon and Gamescom. There are far more demons than there are warlocks with the skill to enslave multiple at one time; things get so bad that the Alliance and Horde are broken for the first time since Warcraft 3 (after one resounding victory after another, both working together and against one another and a mutual enemy in a three-way war), which is why the class orders are revitalized/refounded in the first place.
From a gameplay standpoint, because they're the newest batch of mobs we get quests to go kill.