Apple made this decision just to dump Google and try to gain more control.
Apple dind't want to use OpenStreetMap because its open source solution and Apple never uses open source solution. Yeah, its iOS and OS X is based on Linux (sort of) but they will never tell this to anyone.
iOS 6 Maps sucks just like the App Store on the New iPad. Maybe it is just me but I found it very, very slow. It's better on iPhone