The Ramba”n states that the reason for the downfall of the Chashmonaim was due to the fact that they accepted upon themselves the kingship of Klal Yisroel, which is unique to the House of Dovid HaMelech. The source for this Halacha is from the Yerushalmi which comments on a passuk in Parshas Vayechi. The passuk states, “The staff will not be removed from Yehudah.” Chaza”l learn from here the prohibition of appointing a King from any tribe besides Yehudah.
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Rabbi Aharon Dovid Singer
Rosh Kollel
North Miami Beach Community Kollel