The difference between Jews and Christians in this regard is that Christians were constantly seeking out converts, and thus, always growing, despite Roman oppression. Jews traditionally have not sought converts. Judaism is the national religion of the Children of Israel. It is completely bound up with Jewish ethnic identity. Other than the aberration of the Khazar conversion, there have been few, if any, attempts by Jews to convert others to their faith. Such a religion would be much more susceptible to extinction than Christianity ever was.
Didn't the Hasmonean kings forcibly convert people such the Arabic Idumeans to Judaism?