Well, your contest involves people doing actual work for a chance to win a prize valued at $40. I would imagine that that is less than an hours worth of work for the majority of designers out there. Why would they compete in a project when they can simply save their money and just buy the pack themselves if they want?
If you’re running a competition with a worthless prize entering the competition needs to be easy. Something like commenting on the post or tweeting with a particular hashtag.