I'm not sure what you were getting at with those last 2 sentences. Because, I know some Jews that are Americans, just the same as there are Jews in other countries.
It's the RL bias factor, or perhaps RPing as a Jew or Christian is more appealing? I don't know. I personally followed what my people wanted as their religion until I bought my Great Temple, then I allowed myself the luxury of some RL bias.
Edit: (DAC)Syzygy has good points.