The reasons may have been many, remembering they were imperfect men and any that were trying to hold onto the old law would have done so as that had been all they knew.
For one thing, male circumcision had been instituted by God himself, and it was a sign of a special relationship with him. Predating the Law covenant but later becoming part of it, circumcision began with Abraham and his household. In the Jewish mind, for a man to be uncircumcised was to be unclean, contemptible.
It took time for some to get past and accept what was no longer required
Thus, it required faith and humility on the part of Jewish believers to adjust.
Also, If you read the account in Acts... “Certain men came down to Antioch from Judea and began to teach the brothers: ‘Unless you get circumcised according to the custom of Moses, you cannot be saved.’” (Acts 15:1)
Whether these “certain men” had been Pharisees before converting to Christianity is not stated. At the very least, they appear to have been influenced by that Jewish sect’s legalistic thinking.
Also, they may have wrongly claimed to speak for the apostles and older men in Jerusalem. -Acts 15:23, 24