The HN website works on a phone... but it's hardly where you would want to read it. If you are trying to read the comments, once you're zoomed out enough to see the whole width, the font is usually too tiny to read. And then, to vote on one, you have little tiny buttons that even at the max zoom levels are hard to tap (I've seen quite a few comments apologizing for tapping the wrong arrow on the iPhone).

Now, a platform-neutral website can do what news:yc does too: http://ihackernews.com/ is a good one. But, for the same reason that Twitter clients are popular, some people just like to have one that's built native for their platform.