I'm going to be more cynical than the author and suggest most (but not all) people do this because they want their posts submitted to and getting more attention on HN.
1. I went static, and didn't like third-party javascript solutions
2. People were using comments to contact me or ask questions that were better suited to email
Maybe those are bad reasons, but I've been happy to have one less communication channel.
For example, my blog doesn't have a community of people nearly as strong as Hacker News, so while pushing the discussion to HN is almost always a self-serving act, I don't believe it is always about ego. Often it's about learning.