My hunch is that there is a threshold for social news sites of this general model (voting on items on a "new" page) where the volume of traffic swings the balance of power sharply in favor of upvoting cabals. (Rapid fall of items off the "new" page dilutes the power of the ordinary voting user, but has less effect on the targeted actions of cabals.) After that point, the site's "market" gets severely distorted. Deliberate manipulation by disparate parties all motivated to redirect the maximum number of eyeballs to their site, and this tends to pull excellent sites strongly back towards the average.
This would also imply that upvote cabals can manipulate conditions at a given site in their favor simply by submitting more links.