I've seen competitions delayed ALL the time in all aspects of life. For many reasons too. In this case, it could be that the contest was more popular than they believed and it caused a delay in choosing and preparing a statement for the public to see. It could also be that they planned on giving the winners a more glorious announcement than a simple congrats in the Battlelog forums so they have kept it under wraps until then.
Whatever it may be, just be patient! It's just an art contest for a video game, I'm sure there will be more for you to join in on. :manhappy: