Because that is it's purpose.
I don't think the "purpose" is to prevent implantation. The purpose is to prevent contraception. Which it can only do by preventing ovulation and by interfering with the mobility of the sperm and egg and the ability of the sperm to reach the oocyte and penetrate the zona pellucida. Progesterone does work this way. However, post-fertilization progesterone encourages implantation.
http://www.woomb.org/bom/science/physiology.html