As a long-time coder, software designer, and (now) project manager, I can only say that this is gross mismanagement at hand. Coders are not to blame, and Diablo is not exactly the hardest game in the world to program.
Now, now, let us not judge when we don't have all the information at hand. We cannot call it gross mismanagement without knowing the situation. We can't just assume they've had a properly sized team working full-bore o this project for 12 years.
Let us think about the situation. Just over a year ago, World of Warcraft: Cataclysm sold 3.3 million copies in a single day. That's over a hundred million dollars sales in a single day. And, presumably, they make tens of millions a month in subscription fees after that.
So, Blizzard's raking in hundreds of millions of dollars a year. Now, we don't know their overhead, so we can't gauge how well they're actually doing, but step one in not being grossly mismanaged is being able to rake several hundreds of million dollars a year, no?
It seems to me that management decisions made to make sure that happens (like, say, not splitting the company's attention onto different products, not competing with themselves, and so on) may well be justified. We cannot tell without knowing a significant chunk of business information.