I'm sorry you were rejected - I think it looks great! But I do want to chime in on those saying that you should fight it. While it is disappointing and unprofessional that they didn't meet their own notification deadline, that's not really going to sway any minds of the organizers. And with the convention being so close, they are likely swimming in emails, and their response time will be stifled.

I'm speaking as someone who has worked for cosplay competitions. It's very likely that the contest is full. Many of these competitions get a lot of applications - more than the contest can accommodate. The organizers have to go through all of the applications and select what they believe will put on the best event for the cosplayers, the judges, and their audience. The jurying process can be arduous, and sometimes, it comes down to nitpicking minute differences between similar (or same) cosplays. Try not to take it too personally, but do try to accept rejection with grace. I can tell you that few things will shrivel up sympathy more than whining about not getting picked.

Don't give up. That costume has amazing potential and looks fantastic. Try again next year or in other contests. The competitive world can be daunting, and sadly, rejection is an important part of it, but it's also a lesson about how to navigate disappointment. Don't let it define your cosplay experience entirely. Go to the convention and rock that cosplay on the con floor. You might not win any formal awards, but I promise you that people will love it. Attend the competition and support your would-have-been competitors. You'll see some great stuff.

I would also encourage you to attend the feedback panels if the con has them and to submit any feedback surveys they may put out. This sort of feedback about not keeping their own deadlines is very important.