Because the submitter found it interesting. From the guidelines (https://news.ycombinator.com/newsguidelines.html):

What to Submit

On-Topic: Anything that good hackers would find interesting. That includes more than hacking and startups. If you had to reduce it to a sentence, the answer might be: anything that gratifies one's intellectual curiosity.

Wikipedia articles are not uncommon: https://news.ycombinator.com/from?site=wikipedia.org