Several reasons:
It's a commandment and God says the church that is on Earth just before Jesus comes is going to be keeping.
It has to do with the fulfillment of end times prophecy - the Little Horn aka Beast aka Antichrist aka Man of Sin is said "will think to change the times and laws" and teach "for doctrines the commandments of men".
It is amazing to me that you arrived at the keeping of the Sabbath from the scripture above, which says no such thing!
Attempting to bring Christians under the law is the work of Satan, plain and simple!
Keeping the law as a requirement for salvation, is salvation by works. There is no direct command from the Lord or the apostles to keep the Sabbath or to abstain from certain foods, or anything else. There are no rebukes from the Lord in the letters to the seven churches regarding not keeping the Sabbath.
Why do you continue to ignore the scriptures that I provide that, unlike the one you provided above, which address the issue directly, such as the following:
"Therefore let no one judge you by what you eat or drink, or with regard to a feast, a New Moon, or
a Sabbath. These are a shadow of the things to come, but the body that casts it belongs to Christ."
Paul wrote the above because of those who were still trying live according to the law and who would confront believers who had freedom in Christ, who understood that they were no longer under the old covenant. Notice in the verse that the food law, a Sabbath (meaning any and all Sabbaths).
Anyone who keeps the Sabbath or anything else as a means of salvation, will not inherit the kingdom of God. And that because the person who does so is attempting to take credit in part for salvation, but their own works. It takes their eyes of the finished work of Christ and puts it on their efforts.
Jesus doesn't need any help in fulfilling the law, as He's already satisfied it fully and that on behalf of every believer. And NO not being under the law does not mean that we have freedom to break it. We are under a new covenant, where we follow Christ and are led by the Spirit. When we as believer sin regardless of what it is, we confess our sins and He is faithful and just to forgive us our sins and to cleanse us of all unrighteousness. And then you continue in the process of going from faith to faith and being transformed into the Lord's image, which is the fruit of the Spirit. I posted the following in a previous post. Read it and believe it!
"One person regards one day as better [or more important] than another, while another regards every day [the same as any other]. Let everyone be fully convinced (assured, satisfied) in his own mind."
Meaning that, if one person wants to observe a day as special to the Lord, then he can do that. If another considers everyday alike, then he can do that, I happen to be one of those who worships the Lord every day, meditating upon God and His word and not just on a specific day. But when you teach to others that the Sabbath must be kept, then it becomes a requirement for salvation.
I don't know how we are going to make this any clearer to you. Stop trying to bring believers back under the law. We are not going to trade salvation by grace through faith, for the keeping of the Law which Jesus fulfilled. Why can't you understand the full meaning of the following:
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But some believers from the party of the Pharisees stood up and declared, “The Gentiles must be circumcised and required to obey the law of Moses.” So the apostles and elders met to look into this matter.
After much discussion, Peter got up and said to them, “Brothers, you know that in the early days God made a choice among you that the Gentiles would hear from my lips the message of the gospel and believe. And God, who knows the heart, showed His approval by giving the Holy Spirit to them, just as He did to us. He made no distinction between us and them, for He cleansed their hearts by faith.
Now then, why do you test God by placing on the necks of the disciples a yoke that neither we nor our fathers have been able to bear? On the contrary, we believe it is through the grace of the Lord Jesus that we are saved, just as they are.”
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So, the claim by the Pharisee group is that the Gentile believers must be circumcised and made to obey the law of Moses, which would include everything written in the law, including the keeping of Sabbaths, food laws and any other law. And what was the answer? Why do you test God by placing on the necks of the disciples (Gentile believers) a yoke (the law) that neither we nor our fathers have been able to bear? The council just wrote a letter to the Gentile churches to abstain from eating blood and abstaining from sexual immorality and that was it! And that was in answer to the question of being circumcised and obeying the law of Moses.