The obvious thing that comes to mind is that the general prominence of the competitions could be better.
Can we highlight the voting thread around the forum somehow? Is the 'popular threads' side bar completely automated according to how many replies they've had, or could particular threads be stickied to the top of that? Sure, it will appear in the 'recent posts' side bar too, but that starts a fair way down the page and changes far more often.
Put the voting thread itself in a more prominent place further up the home page?
Make it clearer in the forum rules that competitions are open to everyone to take part in, whether it's to watch and vote, or submit an entry, or both?