I can only assume that the decline in popularity leaves them no choice but to do a complete refresh of a new game to bring people back. Doing this may actually kill the original off or maybe it'll offer enough differences that both games can sit side by side. It means they can also be paid off once again. A new expansion probably wouldn't have been enough to bring people back or sell enough so a brand new game of a dying brand by rejuvenate it.
Having said that, this being Activision Blizzard; I bet it was originally an add-on but they've decided it sell it as new for the ultimate cash grab.