Because it was horribly written, even for a random blog post; and because the time spent reading it could have been better spent reading the Wikipedia article, which would have been both more informative and more pleasant.
Basically, the author wrote a rehash that is worse than its source in every possible way. The only reason to do that is when you're forced to do it, as with an essay in school, or, in this author's case, an arbitrary requirement to write something for Pi day. But that's still no reason to post it on HN.
Well, because his conclusion was basically "shit, I don't know, something to do with sine waves and SHM I guess".
I'd explain why here but it wouldn't fit in this margin.
It seems like most people on HN were expecting something very different, and so were disappointed when it was "just" a bit of history. As someone with a backbround in physics, but who had never heard this tidbit, I thought it was pretty neat.