The simplicity of this piece works in its favour when you consider its purpose. I enjoyed the lack of instruments at times. It's well produced and the atmosphere comes across well. The sound design was definitely a highlight of this track.
However you must be wary - this type of music can often come across as "background" music and in a competition people are looking for the "full package", a piece of music you can listen to on its own. This might not have been a suitable submission and may explain why you didn't do as well as you might have hoped. Food for thought?