Because the law was given through Moses. Grace and truth came to be through Jesus Christ.

Philip finds Nathanael, and says to him, We have found him of whom Moses in the law, and the prophets wrote: Jesus the son of Joseph of Nazareth.

And just as Moses lifted up the serpent in the wilderness, so must the Son of man be lifted up,

Think not that I will accuse you to the Father. The man who accuses you is Moses, in whom ye have hoped.

For if ye believed Moses, ye would have believed me, for that man wrote about me.

Jesus therefore said to them, Truly, truly, I say to you, Moses did not give you the bread out of heaven, but my Father gives you the TRUE bread out of heaven.

Did not Moses give you the law, and none of you does the law? Why do ye seek to kill me?

Moses has given you circumcision (not that it is from Moses, but from the fathers), and ye circumcise a man on a Sabbath.

If a man receives circumcision on a Sabbath, so that the law of Moses may not be loosed, are ye angry with me because I made a man entirely well on a Sabbath?

Now in the law, Moses commanded us such women are to be stoned. What therefore do thou say about her?

They reviled him, and said, Thou are a disciple of that man, but we are disciples of Moses.

We know that God has spoken to Moses, but this man, we know not where he is from.