Just a guess, but probably because Wikipedia does not contain AdSense, so there is no money in putting it to the search results. No one would officially admit

Just a guess, but probably because Wikipedia does not contain AdSense, so there is no money in putting it to the search results. No one would officially admit this, of course, but whenever I see that Google prioritizes X over Y, X is usually a type of thing more likely to contain AdSense than Y.