Malbim: Edom always chose a king from a different land. They did not have a king from themselves, nor a Melech ben Melech. There were always 11 chiefs from Bnei Esav who conducted the Medinah with the king at their head. When Moshe wrote the Torah, Hadar was still alive, and the 11 chiefs mentioned here and there - "Az Nivhalu Alufei Edom." Moshe prophesized that once there will be a king from Yisrael, Edom's kingdom will cease. When this one rises, this one falls - "u'Le'om mi'L'om Ye'ematz." Ezra taught that after Hadar, their last king, died, Yisrael conquered their land and Yehoshua ruled over them. After this, they did not have a king, only the chiefs ran the Medinah, until the days of Yoram.