Because the legal system and the priesthood were one in the same. The law being fullfilled in Christ removed the legal aspect and left only the spiritual. Moses' seat is occupied by Christ. That isn't opinion, that is what Christ spent the latter part of his life telling those that would hear him - both himself and through the Apostles. You might try actually reading what scripture says instead of parroting your philosophers.
The Pharisees weren't priests, and came into existence long after the Decalogue. Nice try, though.
Moses' seat is occupied by Christ.
Why are you demoting Christ? What has he done to you to deserve such ill-treatment?