I remember this contest. I didn't get as enthused about it at the time...
About the only difference that I can see between yours and the winning designs was an option of color. On a lark, I kept your black border and perspective shot as is, but I inverted the black-lines to white-line isometrics.
The parchment paper actually gave the illusion of space to a degree... I wonder if that would have been a more favorable presentation to make yours a winner??
Maybe I can help here's a [link] to a tutorial I made awhile ago. Some people have found it useful. It deals about how to design a mech, but the basic principles can be applied to all sorts of design- themes. Good luck!