Because this was the practice of idolaters, 1 Kings 15:13, and might be an occasion of reviving idolatry. See Judges 3:7 1 Kings 14:23 1 Kings 16:33, 1 Kings 18:19.
Related Commentaries of Deuteronomy 16:21
Deuteronomy 16:21
21 Thou shalt not plant thee a grove of any trees near unto the altar of the LORD thy God, which thou shalt make thee.