Now, this may sound cynical, but what's the betting that this is just Actiblizzard (or whatever they call themselves nowadays) wanting a quick profit?
They've probably sat in the board room and tried to decide what will make them a quick buck. So they decide to make a sequel to Diablo II. Based on it's popularity, they could grab any game and put 'Diablo' and 'III' and people would probably buy it.
That's all I can see here - a marketing ploy, which I assume Actiblizzard are expecting people to fall for immediately.
They probably want more capital for another WoW game.
And, in case you couldn't guess, I'm not a huge fan of the Diablo series.