Writing a good comment takes effort. Interesting posts are upvoted just so that they can be part of a discussion even if you are not the one starting the discussion. I do this all the time. If I feel that I can add to the discussion, I will write up a comment later.
People won't comment if they feel they don't have anything of substance to add - sometimes submissions can stand on their own pretty well and thus don't solicit a lot of comments. It's about maintaining a high signal-to-noise ratio. If anything I've found that you can use the karma/comment ratio as a moderately reliable indicator of link quality.
As for your original question, you should probably "lurk moar" before starting off-topic discussions. This really isn't that uncommon.