Why is the law and legalism powerless to free people from bondage?

The law and legalism are powerless because they are weak and beggarly elements, a dust-groveling system that begs for mercy. They have absolutely no power to lift people out of bondage or paganism, so turning back to them after being known by God is pointless and futile.

Les explains that these believers had come out of paganism and no longer worshipped wood and stone, and now they were known of God personally as His children. Yet they were turning again to the weak and beggarly elements. Les defines beggarly as groveling and begging for mercy. Those things were weak and had no power to lift people out of bondage and paganism. He also compares this to Hebrews, where the old things were good, but now believers have something better, a thousand times better. Since they are heirs with God and have the Holy Spirit to direct them, turning back to a beggarly, dust-groveling legalism makes no sense.

Galatians 4:9

But now, after that ye have known God, or rather are known of God, how turn ye again to the weak and beggarly elements, whereunto ye desire again to be in bondage?

Why could the law and legalism not free the Galatians from their bondage and paganism?

The law and legalism could not free the Galatians because they are weak and beggarly elements, utterly without power to lift anyone out of bondage or paganism. Legalism is a dust-groveling system that only grovels and begs for mercy; it cannot provide the solid standing, sonship, or Holy Spirit's direction that God's people now possess.