When you entered, you gave the rights to your design to Warner Brothers, who may use your design for furture merch or publicity purposes, to change at will. This is regardless if you win a prize or not.
Of all entries, 20 will be selected by fans.
Of the 20 entries selected by fans, WB will pick the 5 that they will make into tees, the ones which suit their needs the best, (which may leave out fandom in-joke designs, cartoony designs, single designs of characters other than Sam and Dean) they do want ‘supernatural’ and ‘join the hunt’ on the shirts somewhere, and when you entered your design, you’ve given them the right to alter it.
The prizes awarded are $750 from MassCanvas, with $250 in merchandise. If you can’t provide a large high-rez copy of your design, it might become ineligible.
The point of the above mentions is that if your design isn’t chosen, it’s not because it’s not good. Or not appealing to other fans. The corporations that create merch have to promote their show, and reach the widest audience possible.
once the contest ends, see if it really benefited you to enter, you aren’t being paid what an artist would who is hired directly by WB, you can’t put this is your portfolio as a ‘client’, if you didn’t win, you can’t produce your work for friends or other fans, for the winners, will they get a boost in name recognition? Time will tell. Next time I hope they just approach fanartists directly and offer them prof pay/rights, that’s really respecting your fans and their creativity.