Why? What makes the new MB a "must have" compared to the Air? As far as I'm aware they shaved a millimetre or two from it's belly, gave it a Retina screen and took away some very handy ports...
Is the new MacBook, it is as simple as that. You need to remember your teen days and that need for having the latest thing. We're talking about Apple here, the most valued company in the world with a fanbase like no others. When Apple releases a new product you better bet people is gonna buy it.