Apple replaced Google Maps in iOS 6 with its own creation. Apple Maps is though full of bugs and misses features that Google has in their maps app. A new rumor says that Google is working on an iOS 6 Google Maps app, but its still months away.
Apple got a lot of criticism for the Apple Maps app shipping with iOS 6. iPhone 5 users around the world found numerous errors in the map data and some users miss features like transit navigation. According to the New York Times Google is working on an iOS 6 Maps app, but it is supposed to be finished at the earliest end of the year.
My assumption is that by then Apple has fixed Apple Maps and the number of users that will download Google Maps app for iOS 6 will be not dramatic. If Google would release the new Google Maps app today it would draw a huge user base. Google should have thought about that and prepare the Maps app earlier.