John Gill Commentary 2 Kings 12:3

"Howbeit the high places were not taken away; the people still sacrificed and burnt incense in the high places." — 2 Kings 12:3 (ASV)

But the high places were not taken away

Used before the temple was built, or set up in Rehoboam's time, (1 Kings 14:23) contrary to the law of God, which required that sacrifices should only be offered in the place the Lord chose to dwell in, (Deuteronomy 12:4Deuteronomy 12:6) the people still sacrificed and burnt incense in the high places; as they had done in the times of Athaliah, and though the pure worship of God was restored at Jerusalem; and indeed this they did in all preceding reigns; nor was it in the power of the best of kings, at least they did not think it safe to attempt to remove them till Hezekiah's time; so fond were the people of them because of their antiquity and supposed sanctity, and for the sake of ease.