Warcraft 3 is the greatest game of all time. But i do not think Arthas story would work as good if it was like a basic action-rpg with one protagonist.
RTS with RPG elements allows devs to switch between personal stories and global conflicts. Jump between characters and build whole world in a meantime.
And it’s extremely disappointing of our industry that almost no one uses RTS for storytelling. Only example that comes to my mind of recent games of this type are the Spellforce 3+addons (it’s really that good, even troll standalone was great).