An enlightening comment is made on this point by Bible scholar Albert Barnes, in his Notes, Explanatory and Practical, on the Acts of the Apostles (1858). Making reference to Acts 21:20—which reads: “After hearing this [an account of God’s blessing on Paul’s ministry to the nations] they began to glorify God, and they said to him: ‘You behold, brother, how many thousands of believers there are among the Jews; and they are all zealous for the Law’”—Barnes remarks: “The reference here is, to the law respecting circumcision, sacrifices, distinctions of meats and days, festivals, &c. It may seem remarkable that they should still continue to observe those rites, since it was the manifest design of Christianity to abolish them. But we are to remember, (1.) That those rites had been appointed by God, and that they were trained to their observance. (2.) That the apostles conformed to them while they remained in Jerusalem, and did not deem it best to set themselves violently against them. [Ac 3:1; Lu 24:53] (3.) That the question about their observance had never been agitated at Jerusalem. It was only among the Gentile converts that the question had risen, and there it must arise, for if they were to be observed, they must have been imposed upon them by authority. (4.) The decision of the council (ch. xv.) related only to the Gentile converts. [Ac 15:23] . . . (5.) It was to be presumed, that as the Christian religion became better understood—that as its large, free, and [universal] nature became more and more developed, the peculiar institutions of Moses would be laid aside of course, without agitation, and without tumult. Had the question been agitated [publicly] at Jerusalem, it would have excited tenfold opposition to Christianity, and would have rent the Christian church into factions, and greatly retarded the advance of the Christian doctrine. We are to remember also, (6.) That, in the arrangement of Divine Providence, the time was drawing near which was to destroy the temple, the city, and the nation; which was to put an end to sacrifices, and effectually to close for ever the observance of the Mosaic rites. As this destruction was so near, and as it would be so effectual an argument against the observance of the Mosaic rites, the Great Head of the church did not suffer the question of their obligation to be needlessly agitated among the disciples at Jerusalem.”
All Scripture Is Inspired of God and Beneficial. 1990. p. 293
Notes, Explanatory and Practical, on the Acts of the Apostles 1859 edition