I think this is simply a function of the perceived value of a story. If it doesn't have many upvotes, readers are unlikely to click on it, and therefore unlikel

I think this is simply a function of the perceived value of a story. If it doesn't have many upvotes, readers are unlikely to click on it, and therefore unlikely to upvote themselves. Likewise, if there has been no commentary on an article, a reader is much more likely to comment on the article's site or not at all, rather than try to force a discussion. An interesting question branching off of this is the percentage of upvotes related to the number of votes for a story. That is, how does the ratio of visits to upvotes change as a story gains traction. In my own personal experience, I don't think there's much of a difference. The only way to know for sure is to get the stats from PG.