With the due respect, questioning rejections is a silly thing to do. If you go for a job interview and you're not hired, do you e-mail them asking why? Or if you submit a photo to a competition and you don't win, do you ask them why they didn't pick you?

Like it was said here, it's a subjective process and they are under no obligation to accept anything from you. They know what's best for their site much better than you (sorry but it's true). Learn from what they are telling you instead of getting defensive. You didn't dump THEM, you dumped all YOUR future potential on that site.

Piece.