I generally don't buy into conspiracy theories, but someone suggested this in my raid last night and the more I think about it, the more sense it makes.
It's the first downtime of this magnitude on a non-patch day that I can recall in recent memory. And it just happens to coincide with a massive marketing campaign for Hearthstone. What better way to get millions of eyes on HS yesterday than to keep WoW offline for a few extra hours?
I know I saw at least 12 people on my Bnet friends list "trying out" Hearthstone for the first time. Extrapolating that effect over the entire population, Hearhstone probably saw a massive spike in traffic - hundreds of thousands of new players at least giving it a shot.
For Blizzard, a cheap (free) and massively effective marketing tactic. The only minor cost is a few pissed off players, and that's something they're used to putting up with anyway.