Because in those last few moments of Jesus' life, God was still dealing with mankind as under the Mosaic Law, which Jesus was in the process of fulfilling. The malefactor demonstrated his utter complete trust in Jesus as God, and in prayer appealed to Jesus as Master, and immediately received the promise of being saved, alive in soul and spirit, with Jesus, in Paradise. Clearly, since this affirmation was under the Old Covenant, a baptism of water was not desired or required.
But, contrary to the current misunderstanding of what the purpose and effect of what the water baptism is and does, it was never intended for, nor has it ever effected the accountability, forgiveness, cancellation, dismissal, nor cleansing of a human's sin-debt to God. To connect it with sin in this way is utter foolishness.
Scripture addresses several baptisms. One is the baptism of John, there is the baptism of the Holy Spirit, the baptism of fire, and a baptism of water.
The question then is, what is the sphere in which water baptism is a matter of concern and of use? Of the various baptisms, the one under consideration is the sphere of of discipleship. And in that sphere, the only functions of water baptism are:
(1) to validate the verbal profession of the subject of his/her allegiance to Jesus as Master, and
(2) a public rite of induction conferring full membership of a heretofore provisional disciple into the local visible Company of The Committed Ones, by the authority of the Godhead as vested in one of the elders of that local Company.
The rite of induction by baptismal immersion in the physical sphere neither effects the forgiveness of sins nor, though spiritual regeneration of the subject is presumed to be a prerequisite, is it proof of spiritual rebirth.
Obviously, the officer of the Company of the Committed cannot forgive the subject's sins nor impart a new spiritual nature to the subject. That is a function reserved to God in the Spirit alone, as shown in the Holy Scriptures.
Pragmatically speaking, this confirmation of induction of a member into the visible Company of Christ-followers, and subject to its authority, the inducted member could be one who only evinces loyalty to Jesus, but has not actually received spiritual rebirth. In fact, None of the initial members of this group had yet been reborn, and actually one of the selected disciples (Judas Iscariot) was a devil, known so to be by the Master Rabbi, Jesus, but nor by his companions. And one other disciple was easily subject to influence by Satanic suggestion. Furthermore, all had very little faith until after the crucifixion of Jesus.
Through history, the membership of the visible local churches has been impure, being a company of professedly "saved" believers, but really being a mixture of both regenerated and unregenerated human souls, a field of wheat and tares, a basket of good fish and bad fish, though they are all of the Kingdom of Heaven and all call themselves "believers.".
In contrast, the human complement of the Kingdom of God and of His Christ is comprised only of faithful friends saved by faith from the Beginning until the time of Jesus' Ascension, and also the souls of truly regenerated believer-disciple-priest-friends of Christ subsequent to the coming of the Holy Ghost in the Spirit-baptism of the first Jerusalem Church.
This Kingdom is not of this visible world. Its locus is in Heaven. It is entered now only by rebirth in The Faith by faith alone, apart from any achievements of the individual of any kind. It only involves a decision based on understanding of the transaction offered by The God: the entirety of one human's sins and sinfulness in exchange for accreditation of all the righteousness of His son, of trading one's soul for everlasting life and the friendship of the Father.
Faith used to be tied a lot more tightly to the ministry of a church than it does today. The standard way of coming to believe used to be at the witness of the church which generally had handy arrangements to do a water baptism with little or no delay. Now it frequently happens at the witness of the bible. Delays in water baptism are frequent today. Mine took over a year from my time of belief before I came to a congregation where I had a stable enough relationship to do it. I remember it because I did it on St. Patrick’s Day. I didn’t wear green, though now I have a bible promise that even when I grow old I will be “full of sap and very green.” Some people think I’m sappy today.
O’Redneck
Eloquent ethos, with the faith that baptism both properly requires and expresses being that which appropriates justification, on Christ's account, "To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus," (Romans 3:26) God purifying the heart by faith even before baptism, (Acts 10:43; 15:7-9) although that can be the occasion when one comes to faith, and a catalyst for it. Since the only kind of faith that is salvific is the kind which effects obedience (all that we do is a result of what we really believe, at least at the moment), confessing Christ, then therefore salvation is promised to those who confess the Lord Jesus, (Mt. 10:32) to those who repent/believe and are baptized, (Mk. 16:16; Acts 2:38) since the former results in the latter, and ideally testifies to true faith, but with belief being the actual salvific means, thus the lack thereof meaning damnation. (Mk. 16:16b)