I think it has to do with how primitive or instinctive the emotion is. I'd bet that fear is one of the oldest emotions, after all every animal experiences fear as far as I know.

Many of the other emotions are restricted to apes, specially humans. I'd say a dog can't be happy or sad, just excited or not excited. If those human emotions evolved thousands of years apart there is a good chance that they are at different parts of the brain.