Hypothesis: In the early days of the internet, people used the search engines their geek friends recommended, which meant that the best search engine (Google) became the most popular. Now computers are better established, and people use the search engines that come bundled with their computer/browser or the ones they're used to using. It'll only be smart geeks (who don't click on ads) that try out new search engines (just like they're the only ones that try out new browsers, like Chrome).