Likely because of verses like this:Then why aren't Christians Jewish? I don't get it
and this:(Rom 3:28-30 NASB95) For we maintain that a man is justified by faith apart from works of the Law. Or is God [the God] of Jews only? Is He not [the God] of Gentiles also? Yes, of Gentiles also, since indeed God who will justify the circumcised by faith and the uncircumcised through faith is one.
Notice that Paul writes to the new gentile believers (Christians) that receiving circumcision (the first major requirement to be religiously jewish) severs the believer from the benefit of the cross (Christ crucified) and enslaves the gentile to judaism. The whole point of the message of the cross (believing Paul's gospel message of Christ crucified) was to free that person from that very thing.(Gal 5:1-3 NASB95) It was for freedom that Christ set us free; therefore keep standing firm and do not be subject again to a yoke of slavery. Behold I, Paul, say to you that if you receive circumcision, Christ will be of no benefit to you. And I testify again to every man who receives circumcision, that he is under obligation to keep the whole Law.