John Gill 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)

Only the people sacrificed in high places
On the tops of their houses, on hills and mountains, and particularly at the high place in Gibeon, where the tabernacle was:

because there was no house built unto the name of the Lord until those
days ;
to which they were obliged to repair as afterwards, and there offer their sacrifices, as the Lord had commanded, (Deuteronomy 12:5Deuteronomy 12:6) .