Because the Surface PRO runs full desktop Photoshop, Visual Studio, Office, Hyper-v and Skrim.

You can also plug in a full USB keyboard and mouse (the OS supports a pointer), use a digitizer pen or an XBox controller.

Because people like myself cross-shop the Surface Pro against compact ultrabook-class laptops. I have opted to go with Surface Pro because of the added flexibility of its design versus this group I consider to be its competition.

I did not cross-shop with iPad and Nexus 10.

Because the Surface Pro's specs match or surpass? a macbook air. It just so happens that it's a PC in a tablet hybrid form factor.