The continued strength of Google as a search provider is based on a large number of factors and won't be debated here, save to say, they have successfully provided the results people are looking for in a manner those searchers either enjoy or are comfortable enough with not to switch to a different provider.
Google continues to tweak their search algorithm multiple times per month and adjust the layout of their results to test for improved visitor experience and advertising revenue.
The majority of Google's revenue is derived from their AdWords and AdSense programs. In fact, advertising accounts for more than 95 percent of Google's earnings. If there is a weakness in the Google model this is it; they need to tweak their layout and results to promote the paid avenues of their offerings. This gives advantages to other engines who may have revenue generation strategies outside of search.