Because the poster thought it might be interesting to those who frequent the site? Considering its position on the front page at time of writing I would say he thought correctly.

How long have you being here? Random Wikipedia article makes its way to the home page from time to time. Every day, ~5 Wikipedia articles is submitted to Hacker News (https://news.ycombinator.com/from?site=wikipedia.org), some have generated a lot of interesting discussions in the past.

Guidelines - what to submit: https://news.ycombinator.com/newsguidelines.html

One of the reasons I visit HN so much is precisely because of these "off topic" articles. Very often they're either interesting, thought provoking or just plain odd/unusual. That's a great thing! I have a wide range of interests so anything that tickles my curiosity is cool. Often, I find the comments far more illuminating and interesting than the original article itself.

Long may these random articles continue!

Things that become popular on HN are pretty much legitimate & on-topic by definition. The guidelines say "anything that gratifies one's intellectual curiosity" is on-topic.[0] Off topic are "politics, or crime, or sports" but even those can have exceptions.

Posting a "why is this here?" comment is a sure fire way to be downvoted and [dead], after which practically no one will see your complaint anyway. Don't like a story? Flag it. It's the only effective way of lodging your opinion on these things.

[0] https://news.ycombinator.com/newsguidelines.html