Because somebody posted it and others found it curious and interesting - among them me. I didn't upvote it, but I found the potential explosion velocity surprising. It may not be of interest to you, but hey, I usually ignore like 50% of the HN homepage right away since I don't care about them. However, I don't go run around and post "why is this on HN?" in those threads.

Because it's a nice physical hack. I personally haven't seen it before.

Also, because some people (myself included) upvoted it.

I have never seen these before. It is nice to see things other hackers consider perfectly normal part of growing up that I completely missed.

The stickbomb is an interesting device, because it is entirely mathematical, and is thus a great hack because normally, you're using a tongue depressor to .. do depressor'y things ...

And by the way .. You are reading Hacker News.

I didn't know it, and I find it highly interesting.

https://xkcd.com/1053/

Maybe this can be the next topic on "What if" of xkcd.