2 Kings 15:35 KJV
Howbeit the high places were not removed: the people sacrificed and burned incense still in the high places. He built the higher gate of the house of the LORD.
He did not remove the high places... Some of those high places were used to worship idols but many if not most of them were used as remote places to worship the Lord.
Following the pattern of the pagans, the Jews practiced the habit of constructing these places of worship on the tops of mountains and in groves of trees. By doing so, they were directly disobeying God's command to worship in the Temple of the Lord. They said they worshiped the living God; they just did it in their own place. It is not unlike those of today who would claim to be worshippers of the Lord but forsake the assembling of ourselves together.
God speaks well of this king but reports the remission. It reminds me of Christ's letters to the churches when He writes "nevertheless I have somewhat against you..." why would a Christian settle for a relationship with the Lord where He still has somewhat against us? Why would we not prefer to remove those "high places" in our lives that are imitations of the world?