We have no way of knowing which of these posts are particularly significant. Many people seem to upvote anything with SpaceX in the title, and I'm skeptical that it should be on the front page so often. Since you say this one is unusual, I believe you.
But really, please send questions like this to hn@ycombinator.com rather than posting them in the threads.
p.s. Downvoting that comment is fine; it's an efficient way for users to communicate that I made a mistake there. That's better than posting a low signal-noise reply.
Edit for clarity.
Google/Facebook/Twitter are on the front page all the time, every time they make some little burp. I like those companies and what they do, but SpaceX is 1000x more interesting to me than any of those.