Can you please explain why some vaccines only work if you have not yet contracted the virus/disease?

Antibodies: The theory behind vaccines is to prevent someone from getting the disease that is being vaccinated against. The way the immune system works for many diseases is that once you have had a disease, your own immune system will prevent you from getting the disease again. Thus you don't need the vaccine any longer. A vaccine allows you to create the necessary antibodies to help you ward off the disease.