Remember, Jehoshaphat was a great king and a good man of God. However, his errors were very costly. Although he loved God, he failed to separate himself from those who didn't share his sentiment (2nd Chronicles 18:1 & 20:35). So, when he set his eldest son (Jehoram) on his own throne, who do you think Jehoram emulated? Well, he acted like Ahab and Ahaziah. That is, he copied the ways of his father's wicked friends (2nd Chronicles 21:6). But that wasn't the worst of it. He killed his own brothers and uncles (2nd Chronicles 21:4), and married Ahab's daughter (2nd Chronicles 21:6). Remember, Ahab's wife was Jezebel. I'm sure her daughter was nothing like her mother though, right?
As you would expect, without God's blessings, things began to unravel rather quickly in Judah. Nations who had been subservient to them suddenly revolted. High places, groves, idolatry and sexual perversion once again became common. After 6 short years, in fulfillment of a written prophecy of Elijah, Jehoram came down with a disturbing disease that eventually caused him to pass his own intestines. Within 2 years he was dead. And notice, when he died, nobody cared. 2nd Chronicles 21:20 says that he "departed without being desired."