Because Nvidia c0cked up really bad. GT 200 is a massive 576mm2, and has huge power requirements and heat outputs. A dual GPU card is not possible on their current 65nm process, and probably still won't be on the 55nm half node. That is why Nvidia will lose the performance crown to ATI when the X2 comes out sometime in late July/early August. The HD 4870 is already 85%-90% as fast as a GTX 280, therefore the X2 will inevitably be faster.
When GT 200 gets to 45nm, that's when we'll really see it shine, but until then...