Google's results were significantly better than other search engines, but only for a while. Soon SEO caught up with it using link farms and other forms of link

Google's results were significantly better than other search engines, but only for a while. Soon SEO caught up with it using link farms and other forms of link spam. Then it was basically the same as old engines which were already spammed to death using other methods. That period of better result quality, combined with other factors you mention, was long enough that most of Google's competition went away. So when the quality went down there was no-one to switch back to.