The old covenant is considered faulty because it relied on human obedience, which was ultimately impossible due to sin.
Hebrews 8:7 explicitly states that if the first covenant had been faultless, there would have been no need for a second one. The old covenant was a typical covenant, merely pointing to Christ and unable to provide true redemption or perfection. Its weakness lay in the fact that it depended on the obedience and faithfulness of sinful flesh, which could never meet God's holy standards. The sacrifices offered under the old covenant could not take away sin (Hebrews 10:1-4); they only served to highlight the need for a better mediator, who is Christ, to fulfill the requirements of God's law and establish a new covenant based on better promises.
Commentary