On Hacker News, the issue is that frontpage space is the scarcest resource that HN has (only 30 slots at a time). Since the site exists for intellectual curiosity, and curiosity withers under repetition, it's important not to have those slots taken up by stories that have had a significant recent discussion. (For example, the OP had a major thread yesterday.)

Reposts are allowed in the other cases. That is, if a story hasn't had significant attention yet, we don't treat a repost as a dupe. Similarly, if it has had a significant discussion but not recently, it's ok to resubmit it. That's because once enough time has gone by, the topic can gratify curiosity again. The cutoff is a year or so. This is in the FAQ: https://news.ycombinator.com/newsfaq.html.