News only lasts 24 hours before it's replaced with the next topic. Most activity occurs on the front page items, as you can see by clicking "new" at the top of

News only lasts 24 hours before it's replaced with the next topic. Most activity occurs on the front page items, as you can see by clicking "new" at the top of this site. But that's people. Spam bots scan sites for open comment forms, and having them on every old page increases the attack surface. Usually the only comments on old pages are tons of spam gibberish. One method sites rely on for anti-spam detection are user reports. But since the humans only pay attention to the new content, the spam is hidden on the old pages, where web spiders index it, and then consider the entire site as spam. So sites will close old comment forms to prevent spam bots from flooding old pages nobody looks at anymore. There are a few other reasons people might do that, like on some sites people get into a heated back-and-forth that lasts beyond the 24 hours, where although nobody else visits that page anymore, 2 people are replying to each other for days or weeks later. By closing it, they're preventing people from doing that.