What's up with not being able to reply to some forum posts?

If there was an announcement somewhere explaining this, then my apologies for asking, but why can't replies be made on some posts? I was trying to reply on something only a few months old and it won't let me. At first I thought maybe it was just my account or something, but then I noticed a few other people saying the same thing.

I just wanted to ask in case the powers that be behind the site didn't know it was happening.