Albert Barnes Commentary 1 Kings 3:2

"Only the people sacrificed in the high places, because there was no house built for the name of Jehovah until those days." — 1 Kings 3:2 (ASV)

The word “only” introduces a contrast. The author is highlighting an exception to the otherwise prosperous conditions he has just described: namely, that the people sacrificed in high places (compare the next verse).

The Law did not forbid worship at “high places” directly, but only by implication. It required the complete destruction of all high places that had been defiled by idolatrous rites (Deuteronomy 12:2). Furthermore, the command to offer sacrifices only at the entrance of the tabernacle (Leviticus 17:3–5) was an indirect prohibition of them, or at least of the way the Israelites were using them. However, it was reasonably questioned whether this command was meant to be in effect before the Lord chose the specific place where the Lord would cause His name to dwell (see Deuteronomy 12:11, 14).

As a result, high places were used for the worship of Yahweh from the time of the Judges onward (Judges 6:25; Judges 13:16; 1 Samuel 7:10; 1 Samuel 13:9; 1 Samuel 14:35; 1 Samuel 16:5; 1 Chronicles 21:26) by people who were entirely unaware of any wrongdoing. God even overlooked this ignorance to the extent that He accepted the worship offered in this way, as is evident from the vision granted to Solomon on this occasion.

There were two reasons for the prohibition of high places. First was the danger that old idolatry would creep back in if its former locations were still used for worship. Second was the danger to the nation’s unity if there were more than one legitimate religious center. In fact, the continued worship at high places ultimately facilitated the division of the kingdom.