Google and Microsoft have never been friends or tried to be from the get go, so the beef between the two companies isn’t a surprise to anyone.
Google Maps worked just a bit on the Windows 8 and WP8 browser, but try it out now and you’ll see it doesn’t work at all anymore. Google’s reason is that the mobile site is optimized for WebKit engine, whereas the IE browser on WP8 uses the Trident engine. We say its more then that.
Update: The news of Google Maps not working on IE9 is sad news so, thanks to improvements too IE9 looks like Gmaps might just work on the browser