Because Rehoboam humbled himself, the anger of the LORD turned away from him, and He did not destroy him completely. Indeed, conditions were good in Judah.
Israel was very much disgraced and weakened by being divided into two kingdoms; yet the kingdom of Judah, having both the temple and the royal city, both the house of David and the house of Aaron, might have done very well if they had continued in the way of their duty; but here we have all out of order there. I.
Commenting on 2 Chronicles 12:1-12
So Rehoboam strengthened himself in Jerusalem,.... Fortified that yet more for the defence of himself, after Shishak departed: and reigned; twelve years more, for he reigned in all seventeen, and this was in his fifth year; of what follows in this verse; see Gill on Kg1 14:21.
when he humbled himself, the wrath of the Lord turned from him--The promise (Ch2 12:7) was verified. Divine providence preserved the kingdom in existence, a reformation was made in the court, while true religion and piety were diffused throughout the land.