Why do you not understand my words ?
Acts 21 is retelling what is written in acts 15 of what to expect of newcomers. They did not live and grow up having Moses preached to them in all their villages so therefore those were the only expectations of them for to come and learn Moses and Jesus preached every sabbath.
Acts 15:19
Wherefore my sentence is, that we trouble not them, which from among the Gentiles are turned to God: 20But that we write unto them, that they abstain from pollutions of idols, and from fornication, and from things strangled, and from blood. 21For Moses of old time hath in every city them that preach him, being read in the synagogues every sabbath day.
Acts 15:28
For it seemed good to the Holy Ghost, and to us, to lay upon you no greater burden than these necessary things; 29 That ye abstain from meats offered to idols, and from blood, and from things strangled, and from fornication:
from which if ye keep yourselves, ye shall do well. Fare ye well.
They did come to the synagogue to learn Moses and Christ every sabbath. Because they had not been raised in the law of Moses they were given commandment to abstain abstain from meats offered to idols, and from blood, and from things strangled, and from fornication, which if they did, they would do well.
For Moses of old time had in every city them that preach him, being read in the synagogues every sabbath day.