Another example of Apple dictating to their users that they must do it the Apple way! I actually think that Apple make reasonably good - if somewhat overpriced - products but that arrogant attitude of forcing users to work the Apple way gets up my nose!
There is more to this issue that meets the eye... this is a summary.
Apple has a contract with Google to provide the map app and map data, this expires the end of this year. If the contact is not renewed, the maps would probably keep working (at least for the immediate future) but it's likely that Google would not allow their maps data to be used in any future iOS versions or on any new devices.
Additionally, Google has done with the Maps app what Microsoft did with Office: put all development effort and new features on their own platform's version first (Android, Windows respectively) and roll them out to Apple's platform when they feel like it. The Android version of Maps app already has voice navigation and many other features, but Google has not made them available to iOS.
Maps are more important on the mobile platforms that anybody expected them to be. Not only is it a functionality issue, but Google makes a lot of money out of the maps advertising through the iOS devices and Apple gets none of it. Apple is essentially hostage.
Apple reasoned that many major change (ie, new maps app) needs to be introduced in a major software release, ie, iOS 6, and not a minor one e.g., iOS 5.2. So they had to jump NOW and roll-out the changes in iOS 6 before the data contract expired.