Because the law was given by Moses; grace and truth came through Jesus Christ.

Philip finds Nathanael and says to him, We have found Him, of whom Moses and the prophets, in the law, did write, Jesus the son of Joseph, who is from Nazareth.

And as Moses lifted up the serpent in the wilderness, so does it behoove the Son of man to be lifted up:

For if you were believing Moses, you were believing me; for he wrote concerning me.

Then Jesus said to them, Truly, truly, I say unto you, Moses did not give you the bread from heaven; but my Father gives you the true bread from heaven.

Did not Moses give you the law? and no one of you does the law. Why do you seek to kill me?

Moses gave you circumcision, not that it is of Moses, but of the fathers, and you circumcise a man on the Sabbath.

If a man receives circumcision on the Sabbath, in order that the law of Moses may not be broken; are you mad at me, because I made a man entirely whole on the Sabbath?

They railed on him, and said, You are His disciple; but we are the disciples of Moses:

we know that God spoke to Moses: but we do not know this One, whence He is.