I don’t know how many times I’ve come across the statement “competition is fierce” in writers guidelines. But it’s really true. If one goes the traditional road to getting published and not the self-publishing route, the competition to get one’s work out there, published in other words, is extremely fierce. This may be the reason why one’s work doesn’t get published, but you have gone as far as doing your homework in every aspect. The work is still rejected. As long as you know you’ve fitted the requirements and then some more, I think the reason for one’s rejection is simply, once again, that competition is fierce.