There was an article a while back that talked about it. There are a variety of reasons somebody would decline the offer of being made. Some don't like the commitment and demands of a made man. It's also easier to be charged with a RICO case since you are an official member of a criminal enterprise. Although there is a rise in people declining the offer, most eligible associates would love to be made. The Five Families have no problem filling the ranks. It's the quality of the ranks that is a little more difficult.