All Things D reported today why Apple switched to their own maps solution from the popular Google app: Google refused to enable voice-guided turn-by-turn directions. A maps app without voice-guided directions is problematic, because you have to take your eyes off the road to look at your phone. I paid $50 this summer for the Tom Tom app and found the voice directions invaluable when I was traveling. All Things D says that Google felt that voice-guided directions, long available on Android phones, was an important distinguishing feature for Android devices and was simply unwilling to make this technology available to one of its partners. Plus, this feature apparently wasn't part of Apple's original contract with Google first hammered out over five years ago. As you can imagine, Apple likes having complete control over its products, and not being able to control this was apparently too much, so Apple decided to go its own way.
All Things D says there were other points of contention, too, including Google wanting more control over the app. They reportedly wanted Apple to brand the App as Google's and to add Google Latitude, both of which Apple refused. Apple had already been buying up mapping companies, and decided it could do without Google. Apple says that as people use their new Maps app it will get better, because facets of it are crowd-sourced. And they're hiring more people and working hard to improve it.