One wonders why baptism was not proscribed for John the Baptist. A truly great theologian of the Reformation tells you. John was the Law, Christ was Grace. Baptism is under Grace:
If man is to become spiritual and a believer, he must necessarily first be under the law; for no one can know his faults without the law, and he who does not know his sin will not long for grace. But the law demands so much that man must realize and confess that he is unable to satisfy those demands. Then he must despair of himself and in all humility sigh for the grace of God. Behold, therefore the seven years come first, the law precedes grace as John the Baptist was the forerunner of Christ. The law kills and condemns the natural, sensual man, so that grace may lift up the spiritual, inner man.
The Holy Spirit indeed convicted the world by preaching before from the beginning (for Christ ever rules, and is the same Christ “yesterday and to-day, and for ever,” Heb13, 8) through the holy fathers, Adam, Noah, Abraham, Moses, Elijah, Elisha, and John the Baptist, and this conviction was upheld by divine power. But now the true beginning is to be made, and Christ will institute a public conviction, which is to be extended not only over the Jewish people, but over the whole world until the last day.
And finally, because John died before Christ, to which we read in Paul – who actually matters:
What shall we say then? shall we continue in sin, that there may be abundance of grace? God forbid. How shall we that are dead as touching sin live any longer therein? Remember ye not that all we which are baptised in the name of Christ Jesu, are baptised to die with him? We are buried with him by baptism for to die: That (likewise) as Christ was raised up from death by the glory of the father: even so we also should walk in a new life. For if we be graft in death like unto him: even so must we be in the resurrection. This we must remember, that our old man is crucified with him also, that the body of sin might utterly be destroyed, that henceforth we should not be servants of sin. For he that is dead, is justified from sin. Wherefore if we be dead with Christ, we believe that we shall live with him: remembering that Christ once raised from death, dieth no more. Death hath no more power over him. (Rom 6:1-9 William Tyndale)