Why was David's house preserved despite Joram's evil reign?

Following the death of King Josaphat and his burial with his ancestors in the city of David, his eldest son Joram became king of Juda. Before his death, Josaphat had given Joram's six younger brothers—Azarias, Jahiel, Zacharias, Azaria, Michael, and Saphatias—great gifts of silver, gold, and pensions, along with strong cities in Juda, while assigning the throne to Joram because he was the firstborn. However, once Joram had established himself over the kingdom of his father, he killed all his brothers with the sword, as well as some of the princes of Israel.

Joram was thirty-two years old when he began to reign and reigned for eight years in Jerusalem. He adopted the pagan customs practiced by the monarchs of Israel, following the corrupt pattern set by Achab's dynasty, for his wife was a daughter of Achab (identified elsewhere as Athalia; cf. 2 Paralipomenon 22:2), and he did evil before the Lord. The Lord did not destroy the house of David because of His covenant and His promise to give a lamp to David and his sons forever.

During those days, Edom revolted from being subject to Juda and established their own king. Joram marched out with his princes and his entire cavalry, launching a night attack that defeated the surrounding Edomites and their cavalry commanders; nevertheless, Edom remained in revolt from the dominion of Juda up to the narrator's day. At that same time, the city of Lobna revolted from under his authority because Joram had forsaken the Lord the God of his fathers. Moreover, the king constructed high places in the cities of Juda, compelling the inhabitants of Jerusalem into spiritual fornication and causing Juda to transgress.

A letter was then brought to Joram from Eliseus the prophet, carrying a warning from the Lord the God of David. The message declared that because Joram had abandoned the upright paths of his father Josaphat and of King Asa, choosing instead the idolatrous ways of the kings of Israel, seducing Juda and Jerusalem after the pattern of Achab's house, and murdering his own brothers who were better men than himself, the Lord would strike him, his people, children, wives, and property with a severe plague. Furthermore, the prophet foretold that Joram himself would be afflicted with a grievous bowel disease until his vital parts came out little by little each day.

The Lord then stirred up the Philistines and the Arabians against Joram who bordered the Ethiopians. These invaders overran Juda, took the property from the king's house, and carried away the king's wives and sons, leaving only his youngest son, Joachaz (identified as Ochozias in the edition's annotation). In addition to these disasters, the Lord struck Joram with an incurable intestinal disease. Two full years passed; after a long wasting illness, he voided his bowels and died in severe suffering. The people did not give him the traditional funeral burnings customary for his royal predecessors. Although buried within the city of David, he was buried outside the royal tombs.