I think this is a case of business strategy conflicting with writers.
Edit:
Didn't we see the same in Starcraft II and Diablo 3 either? They leave options for story in expansions, but they generally like to wrap up the story started at the beginning of each game. In Starcraft II that was the Jim Raynor/Kerrigan story. In Diablo 3 it was defeating all Evil and the triumph of mortals.
I doubt all Blizzard games have the same writers. So I think it's a business strategy.