Here is a comment that garnered almost 800 responses and was voted up 37. Why wasn't it promoted?

Can comments be promoted? Perhaps the commenter should have been encouraged to turn his comment into a top-level post, but a moderator can't just change a comment into a promoted post with the same username. Also it would have split the discussion, so people might have been reluctant to encourage that.

As for people tending to post more in Discussion than Main, I read somewhere that Discussion has more readers. I for one read Discussion almost exclusively.