The reason Google Maps was successful in its early days wasn't because of its quality, it was because it was free.
The other options were just as good, but often you had to pay for them. With Google, you could repeatedly refine the bad results until you found something useful, because there was zero additional cost involved.
Maps had smooth, animated panning and zooming that updated the map in real time without reloading the page. MapQuest and the other major competitors at the time only allowed panning and zooming by clicking buttons on the side of the map, which scrolled a predefined number of steps and reloaded, at least initially, the entire page when you did so.
Google Maps was also less cluttered and used the browser's real estate much effectively. MapQuest was a small, cramped map by comparison.
