Because it's silly to ask web developers to cater to a specific extension used by a marginal minority of people. Much better to use a more general method for when JS is unavailable.
And people using noscript or similar tools (like myself) should know by now when a page fails to check that first.
But the comment applies to anyone who's not using JavaScript in their web browser.
Although I like programming in JavaScript, there's many people I know that consider JavaScript itself to be a silly extension of what a web browser should do.
Hacker News is a fairly simple site for this use case. It works. However I would not say that it is perfectly fine.
Try upvoting with js disabled. The page has to do an entire refresh. That could be a doorstopper for people with limited data plans or with spotty network coverage.
The percentage of people that considering JavaScript "to be a silly extension of what a web browser" is so small that it would be a rounding error in most charts.
Funny, I'd put that in the exact opposite order. Back in the day there were taskbars, popups, visitor counters and what have you, JS was mostly an annoying gimmick. Today it's about actual site functionality, it's pretty fast and (generally) sandboxed.
And people using noscript or similar tools (like myself) should know by now when a page fails to check that first.