Great question indeed, the way I see it antiprogestin is used to terminate where progestin is more of contraceptive.
Though both have so much in common I do find it quite unique how they don't work interchangeably.
Someone please correct me if I'm wrong this is just a guess.
progestins make cervix thick so sperm cant travel up. so i guess u take it before having sex.
antiprogestins terminate pregnancy, which means ur already pregnant and the blastocyst is sticking to the endometrium lining. Progesterone is needed for endometrium to be sticky. if u stop progesterone the blastocyst wont stick anyone and u add misoprostol (contract uterus) to eject the contents out