Because although baptism is the sign of the new covenant replacing circumcision, in
Galatians 6:15 it says circumcision doesn't count for anything but a new creation does.
Thanks
@Sola1517 for your reply. I believe that Paul makes that statement in Gal 6:15 due to the theme of the letter to the Galatians, which has been written predominantly against the false teaching of the Judaizers, who insisted upon the circumcision of Christians. But as you quite rightly point out, baptism corresponds to the sign of circumcision given to Abraham, and also that what is crucial is a new creation.
What I would like to point out is that elsewhere, these same matters are spoken of in relation to baptism, and the new life:
Romans 6:3–4 (ESV)
3 Do you not know that
all of us who have been baptized into Christ Jesus were baptized into his death? 4 We were buried therefore with him by baptism into death, in order that, just as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life.
Colossians 2:9–15 (ESV)
9 For in him the whole fullness of deity dwells bodily, 10 and you have been filled in him, who is the head of all rule and authority. 11 In him also you were circumcised with a circumcision made without hands, by putting off the body of the flesh, by the circumcision of Christ, 12
having been buried with him in baptism, in which you were also raised with him through faith in the powerful working of God, who raised him from the dead. 13 And you, who were dead in your trespasses and the uncircumcision of your flesh, God made alive together with him, having forgiven us all our trespasses, 14 by canceling the record of debt that stood against us with its legal demands. This he set aside, nailing it to the cross. 15 He disarmed the rulers and authorities and put them to open shame, by triumphing over them in him.
2 Corinthians 5:17 (ESV)
17 Therefore,
if anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come.
Note in these texts that in order for us to be the creation God calls for, it is necessary that we are to be found in Christ. The process of baptism is the way in which God has promised this to be the case for those who believe. This is not to deny the power that saves is the redeeming power of God carried out spiritually, by Him putting off the body of our flesh by the circumcision of Christ. But that it is directly in our baptism Col 2:12 that the Bible ties the two together. It is in our baptism that we are united with Christ in His death and resurrection life as a new creature.