Probably because the “Christian” part is rooted in the “Judeo” part. Our Savior is a Jew.
Nobody denies that. Technically, socialism is rooted in Catholic religion as well which is rooted in Judeo-Christian tradition too. I think America deserves more credit than the rest of the Western world which would all be socialist but for us. Even now, politicians in Europe win elections by calling themselves socialist. It wouldn’t work here. So there is something else about America. Ayn Rand was not religious, but her works have sold in huge numbers here.
I don't think that's a defensible position historically, philosophically or religiously.