Wikipedia is a open source webpage which provides knowledge for various subjects. You can even create and edit pages on Wikipedia. Google gives priority to Wikipedia as it provides true knowledge from various users online.
Wikipedia is a very old site and it provides information about each and every people, material, animals, places etc. Also there is no chance to spam with Wikipedia. So that Google gives first priority to Wikipedia.
Wikipedia has huge number of articles contributed by a global volunteer community of more than 100,000 people. Google promotes Wikipedia as the web's center for knowledge. And also, Google Gives $2 Million to Wikipedia's Foundation in Fab,2010.