Albert Barnes Commentary 2 Chronicles 21:3

"And their father gave them great gifts, of silver, and of gold, and of precious things, with fortified cities in Judah: but the kingdom gave he to Jehoram, because he was the first-born." — 2 Chronicles 21:3 (ASV)

Jehoshaphat departed from Rehoboam’s policy (2 Chronicles 11:23), actually giving his sons the fortified cities where they lived. It is probable that this provoked Jehoram’s jealousy and led 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 Exceptions to this rule in the northern and southern kingdoms include Solomon, where divine appointment superseded the natural order; Abijah (2 Chronicles 11:22); and Jehoahaz (2 Kings 23:30).