Albert Barnes 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)

The worship on the “high places” seems to have continued uninterruptedly until the time of Hezekiah, who abolished it (2 Kings 18:4). It was, however, re-established by his son Manasseh (2 Kings 21:3). The priests at that time cannot have regarded it as idolatrous; otherwise, Jehoiada would have stopped it during his regency.