While a contest is still active, designers also withdraw if:
1- they feel through feedback (or lack of) that there is no interest in their design. Many designers hang in there to the end, leaving work up, just in case. Others are impatient and simply move on.
2- if they have submitted work too similiar to another designers previously submitted work
It looks like you're being a great contest holder, giving lots of feedback -- and it's generous of you to be thinking ahead to acknowledging non-winning designs with ribbons. I think a lot of us wish all contests were run with as much participation and consideration as yours!