The main reason behind Wikipedia's high ranking in Google search is that people link to Wikipedia articles. It may seem strange that people choose to link to Wikipedia instead of other sites, but the "free encyclopedia that anyone can edit" has many selling points:
1. Wikipedia articles have a lot of content and people like to link to pages that consist mostly on text. There's a lot of information structured in a consistent way and that makes most of the articles valuable.
2. Wikipedia articles have an accessible language. Even if you're not familiar with a domain, you can understand the basic things and it's easy to learn more.
3. Why shall I link to the Wikipedia page for "Ask a Ninja" instead of the homepage of the videoblog? Because the Wikipedia page start with "Ask A Ninja is an award-winning series of comedy videos about the image of ninjas in popular culture", while the homepage shows the latest videos. There's more context in Wikipedia.
4. Wikipedia pages are more objective than other sites because they don't try to sell products or ideas, they only explain what is all about and include criticism. Another selling point is that Wikipedia doesn't have ads.
5. A Wikipedia article is constantly updated, so you can say it's never finished. You're more likely to link to a page that will be still fresh next year.
6. Wikipedia is already familiar to many people, so you treat as and old friend that has answers for most of your questions. You don't trust other sites enough to link to them, it's hard to decide which one deserves this honor, so you link to Wikipedia.
7. You treat Wikipedia as a free-style dictionary that has entries for a lot of things. So when you need to explain something you link to the extended dictionary.
8. If you link to a Wikipedia article, you don't need to link to other sites because the external links from that article are very good or at least good enough.
9. You've already found a lot of interesting things from Wikipedia so you tend to trust it and recommend it to others.
10. Wikipedia pages compile a lot of disparate information from different sites, so they're a good reference point for someone who wants an overview on a topic.