Jehoshaphat departed from Rehoboam’s policy (2 Chronicles 11:23 note), actually making over to his sons the “fenced cities” in which they dwelt. This, it is probable, provoked the jealousy of Jehoram, and induced him to put them to death 2 Chronicles 21:4.
Because he was the firstborn - Compare Deuteronomy 21:15-17. Exceptions to this rule in the northern and southern kingdoms are Solomon, where divine appointment superseded the natural order, Abijah 2 Chronicles 11:22 note), and Jehoahaz (2 Kings 23:30 note).