Because it

a.) sustains a market for ivory

b.) a lot of ivory has falsified paperwork

c.) enforcement of these rules is costly and there isn't enough people to do it http://www.washingtonpost.com/national/health-science/overwh.... For example, the Fish and Wildlife service in New York has 6 employees which are supposed to be checking all cargo coming to NYC and NJ. That's very few people for the job.