[jzwinck]

It's an overall larger system. A larger sensor means less noise (assuming fewer pixels per inch, which is true in this case). A larger lens means less distortion (because any imperfections are smaller relative to the entire glass).

And it's very expensive, which means even a relatively small advantage in image quality will make the resulting pictures more valuable, at least for a few years until someone else does this again with newer technology. DSLR photographs are truly commoditized now, but medium format are not, never have been, and probably never will be.