it's just the way it is. saving the boxers who already lost from further punishment has something to do with it. in amateur boxing they are quite strict like that (e.g. holyfield in the olympics). if a boxer takes a beating it is unnecessary to put him in the ring the following day. better to give out 2 bronze.
The 28 day stoppage rule. If one semi ends in a KO or RSC that fighter would be handed a28 day suspension rendering it unfair that he/she could not contest bronze.