Two reasons mainly… they try very hard and they adapt constantly.
That’s where the contextual signals come in. All search engines incorporate them, but none has added as many or made use of them as skillfully as Google has. PageRank itself is a signal, an attribute of a Web page (in this case, its importance relative to the rest of the Web) that can be used to help determine relevance. Some of the signals now seem obvious. Early on, Google’s algorithm gave special consideration to the title on a Web page — clearly an important signal for determining relevance. Another key technique exploited anchor text, the words that make up the actual hyperlink connecting one page to another. As a result, “when you did a search, the right page would come up, even if the page didn’t include the actual words you were searching for,” says Scott Hassan, an early Google architect who worked with Page and Brin at Stanford. “That was pretty cool.” Later signals included attributes like freshness (for certain queries, pages created more recently may be more valuable than older ones) and location (Google knows the rough geographic coordinates of searchers and favors local results). The search engine currently uses more than 200 signals to help rank its results
The article was fascinating to this Computer Scientist and I bet you non-technical Google users will find how they do what they do pretty interesting too.