Hacker news traffic lasts just 2 days.
If there's less things posted to /new at that particular time, it seems a bit more likely someone gets a chance to interact with it before it disappears from that first page of /new.
At least HN tolerates reposts if you don't get any attention the first time round [1]; if your launch bombs on ProductHunt, you don't get a second chance (ask me how I know!)
On gsky's about HN traffic lasting two days - that was definitely my experience, though if it's interesting enough it'll likely get picked up by other sites too. Blogs, podcasts etc are always picking stuff up from Reddit and HN.