Moses was in Midian probably for ten years. During this period the former Pharaoh died and another member of the Pharaoh dynasty ascended the throne of Egypt. Then, Moses along with his wife (and, according to the Torah, with two children) left again for Egypt. On the way, he underwent the divine test on Mount Tur (Mount Sinai). God, who directly talked to a man on Mount Sinai, can also directly address all other human beings and apprise them of His will. But this is not the way of God. Direct address means removal of the curtain, while the considerations of human trial demand that the curtain should remain in place. So God reveals His message only to a selected person and causes this message to be delivered indirectly through him to others.