AJAX, especially at the level of Google Maps, is very difficult to get right. If you’ve got a large team and plenty of dollars, it can be done — then you have to worry about the browser. Deploying to Flash eliminates most of these issues, and allows you to go places AJAX has a hard time going: mobile devices.
Ajax does steal some of the Flash spotlight. Being able to do HTTP calls out of Flash movies was always a big selling point for that type of thing.