The old system in the law of Moses was only a shadow of the things to come, not the reality of the good things Christ has done for us. The sacrifices under the old system were repeated again and again, year after year, but they were never able to provide perfect cleansing for those who came to worship.
If they could have provided perfect cleansing, the sacrifices would have stopped, for the worshipers would have been purified once for all time, and their feelings of guilt would have disappeared.
But just the opposite happened. Those yearly sacrifices reminded them of their sins year after year.
For it is not possible for the blood of bulls and goats to take away sins.
That is why Christ, when he came into the world, said, "You did not want animal sacrifices and grain offerings. But you have given me a body so that I may obey you.
No, you were not pleased with animals burned on the altar or with other offerings for sin.
Then I said, 'Look, I have come to do your will, O God -- just as it is written about me in the Scriptures.'"
References for Hebrews 10:7
Christ said, "You did not want animal sacrifices or grain offerings or animals burned on the altar or other offerings for sin, nor were you pleased with them" (though they are required by the law of Moses).
Then he added, "Look, I have come to do your will." He cancels the first covenant in order to establish the second.
And what God wants is for us to be made holy by the sacrifice of the body of Jesus Christ once for all time.