Votes declined when we mandated a minimum number of posts to vote, and made all votes public (although it looks like we got a slight bump this last month).
However, we traded volume of votes for credibility of votes. We had a situation where entrants were encouraging their friends to join and vote. Because this wasn't against the rules, it created an environment where artists increasingly had to "recruit" voters to win. I know this practice is common, especially in Facebook "Like" contests, but, IMO, it ran the risk of transforming the contest from an art contest into a popularity contest.
The upside now is that we have more of a meritorious contest, with a greater variety of winners. If the contest is enjoyable, I see no reason to discontinue it.
If we want the volume of a popularity contest, with the merit of an art contest, one option may be to encourage clicks on the "Nominate Post for Newsletter" link that accompanies the art contest threads.