Their shareholders would like to know this also.
Most (fan) speculation is to compete with WoW. I think they wanted to hook people "bored with WoW" before new content came out - only they didn't have much of a hook. They had a shell of a game with promise it would be better. They should have waited until March (in my opinion) when people had a taste of Cataclysm and are ready to try something new.
It would have been slightly more risky, but they didn't have the product ready for this intent - assuming its what they were going for.
I think 90% or more were pretty shocked at their release date. I think most of us expected Nov or Dec - if not a delay until 2011. I certainly was - and the game was about what I expected when I first logged in. Exactly as the Beta: unfinished and unrefined.