Ask HN: Why does HN have multiple domains?

I can see in my web stats package that I get referrers from multiple ycombinator sites. e.g.

http://news.ycombinator.com

http://news.ycombinator.org

http://news.ycombinator.net

Why doesn't HN have one canonical site? He could redirect two of them to one? (For reference, it looks like YC owns the .com and PG himself owns .org and .net so it's possible).