They're not focusin on D3, they have 2 other games to focus on. The reason why they could do SC1, D2, and Wc3 at the same time was because Blizzard north was working on D2 not Blizzard Entertainment.
Now Blizz Ent. has to do all 3 at once. They'll be releasing SC2 and Cata this year, then D3 will be up to full speed.