Before Aaron was chosen, all of Israel were kasher for the priesthood. Once he was chosen, the rest of Israel were excluded. As it is written (Numbers 18:19) "An everlasting covenant of salt is it (the priesthood) before the L–rd for you (Aaron) and for your sons," and (Numbers 25:13) "And it shall be unto him and to his seed after him the covenant of an everlasting priesthood."