Why is being Jewish considered an ethnicity?

It is considered an ethnicity because the development of the Jewish religion occurred among a specific group of people, and so as the religion grew, so did the ethnicity associated with the religion. The Jewish ethnicity is defined, therefore, partially by ancestry and partially by religion. The Jewish community also stayed closed off to matches not within the community, preserving their ethnical integrity. That is, until the modern day, where one can be an ethnic Jew, a religious Jew, or both.