Google has Search to promote their stuff, even if it's inferior. Put theirs on top, meaning other reviews are much harder to find (people click on top links). That's how many people installed Chrome for example
You think when Google Maps was launched their primary competitors (e.g. MapQuest) had anything comparable? Maps was one of the first, large scale, Web 2.0/AJAX applications. At the time, most other mapping products were still reloading an iframe or the whole page everytime you needed to change the map.
Gmail was lightyears better in both performance, spam filtering, organization, and speed compared to other major WebMails at the time.
The New Google Maps is a quantum leap over competitors like Bing Maps, even native iOS maps.