Wondering why Apple (AAPL) is sinking so much effort into building its own Maps application? Because it doesn’t want Google (GOOG) to gobble up all the revenue from big-name mobile applications. ComScore has published its most recent monthly review of the top iOS and Android apps in the United States ranked by unique visitors and has found that Google captured 5 of the top 6 spots with Google Maps, Google Play, Google Search, Gmail and YouTube. In fact, Facebook (FB) was the only non-Google app to crack the top 6, although it also had the benefit of being the most-visited app in the entire country by a margin of more than 10 million unique visitors. iTunes was the only Apple app to crack the top 10, meanwhile, as it ranked eighth with roughly 46 million unique visitors last month.