As King, the Lord Jesus Christ instituted new forms of administration for the two sacraments given to His church. The oId Testament forms looked forward to the Messiah's death; they prefigured this by requiring the shedding of blood. The New Testament forms looked back upon the finished work of Christ and the full blood-price having been paid by Jesus' death; they therefore require a non-bloody form of administration. Additional bloodshedding would deny that the full debt had been paid.
New Testament baptism pictures separation from the world and entrance into a living relationship with God - into His covenant and church - as circumcision pictured in the Old Testament. While this separation from the world and relationship with God has both an external and internal form, the deepest meaning of both circumcision and baptism is the inward, spiritual one - the cleansing from sin by the blood of the Savior.
After Jesus' death, why were new forms required for the Old Testament sacraments?