I'm sure there's a number of reasons.
Romans 3 Paul explains about being Jewish and how faith and works go together. Faith saves but works is a demonstration of that faith. In Romans 3:31 he shows that the Law is still integral in the Christian walk. All of the Apostles were Sabbatarians. Did they preach on the Sabbath only? No, but they kept the Sabbath day holy because that is Gods Law. Gods Law does not save but only points out sin. It is a measurement to judge how far from perfection we are and how far we have to go.
It is impossible to follow the Law without a new heart. You must want to please God and not yourself or your Church friends. That is where the Holy Spirit is to come into your life to help fulfill Gods Law as He wishes. That's what the New Covenant is all about.
Proof by blatant assertion? We don't know how the Apostles kept the Sabbath. We do know that Ignatius of Antioch, who knew Peter, Paul, and certainly John, wrote in his epistles (on his way to martyrdom) congratulating his readers on "no longer sabbathing". Paul opposed imposing Jewish ceremonial practice on Gentile Christians, and that's exactly what Sabbath-keeping is.
Paul very clearly taught that there was no more distinction between Jew and Greek in ethnic terms. That both are now part of the family of God - the greater Israel.
And, no, the Apostles were NOT Sabbatarians - they worshipped the Lord on the Lord’s Day, the FIRST day of the week.
Paul writes that the Sabbath is a shadow of things to come, and that the substance is in Christ. It is clear from this text that Paul, like the Old Testament writers, considered all the Old Testament convocations as inseparable; indeed, in saying that all three are a mere shadow of things to come, he makes no distinction between the first two terms and the third. Paul concludes that the reality lies in Christ. The Greek literally reads: “but the body is of Christ,” meaning that all of our lives and all of our energies need to be submitted to Christ who is ever present to us and that the old covenant convocations such as the Sabbath are no longer binding.
In Paul’s letter to the Romans, written around 57–58, he says, “For one person considers one day more important than another, while another person considers all days alike. Let everyone be fully persuaded in his own mind. Whoever observes the day observes it to the Lord” (Rom. 14:5–6). The apostle is speaking here about the day which is being observed to the Lord, i.e., the day of worship. He notes that this is up to each person to decide. It must be noted, however, that Paul does not specifically mention the Sabbath here.
From these texts it seems clear that Paul considered Sabbath observance a matter of personal conviction that was not important in itself. Moreover, since Paul was presumably responding to the churches in Colossae, Galatia, and Rome about matters which concerned them, it seems clear that some Christians were worshiping on days other that Sabbath in Rome and in Asia Minor around 54–58.
John wrote his gospel in 80 to 90 AD - John wrote how the world had been symbolically created anew in Jesus. One implication of this is that with the passion, death, and resurrection of Christ one eternity had ended and another had begun. God could therefore abrogate an everlasting law and still not contradict himself.
There is widespread belief among Christian scholars that the institution of Sunday worship occurred in the apostolic or post-apostolic age in commemoration of the Resurrection.
"Upon the first day of the weekσαββατων let every one of you lay by him in store, as God hath prospered him, that there be no gatherings when I come" (1 Cor. 16:2 AV).
The Corinthians were commanded by Paul to meet on the first of the week and have their tithes ready to be gathered each week.
The whole week is a heptad, a bundle of seven, a week of days, a "sabbath" of days, wherein the last day for Jews was set aside for rest from work. But the evenin of that last day is the first day of a new sabbath of days, hence it is also a (new) sabbath.
If you are a sabbatarian, avvording to the law you musy stay at home, go no place, and do no woek. But also if you wish to thus come inder the Law, you must obey the law, all of it, without fail. And doing so will send you to Hades/Hell in it in any way. You cannot be a Christian and be a sabbatarian.