Martti, that's too bad, but don't feel badly about it. The problem with any art contest is that it's hard to judge objectively, and unless the artists have a way to know what the judges like, they can't slant their work to the judges' tastes.
If I'd been a judge there, anything else would have had to be super impressive to score higher than yours (on artistic merit - on subject, only something else based on SSSS could compete)(see what I mean about the difficulty of being objective? )
I know, well, the worst in artcontest when I knew that I was literally the last one, even my art looked pretty good., however in that the people can vote and random people usually vote for a fandom they like, not for the art, so this contest is better then the other, which I never entered since then.
(there are two type of contest, one where I send a pic weeks before the con and people can vote for the winner and maybe there is judges too.... and then there is the one which is at the con I need to draw for a theme in 2 hours. and here the judges chose the winner)
I am a bit disappointed, but I am happy with the result of the art, I did it easily in 2 hours, and didn't have to hurry like any other times before.