The main reason? I stink at short story writing. No, truly. I think it takes an incredible level of skill to tell a worthwhile, interesting, layered story in a terribly-brief amount of time. Much like those short films I'm showing in class right now ("Paris, I Love You" is the title) - 5 minutes to tell a story with punch, with impact, with flavor. I don't have those skills, but I soooo admire those who do. I personally need about 400 pages to tell my stories, lol. Seriously - I just love the freedom of having that many pages to play with.
Sure, I've attempted the odd short story here and there. But I don't enjoy them, probably because they're so challenging for me. So, I'll leave short story writing up to the ones who do it best.
Good luck to all who enter the contest!!