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.
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 ...