History

Judaism is what's called an ethnic religion, rather than a universalizing religion. Ethnic religions generally don't seek converts. There are accounts of forced conversion by Jewish fundamentalists in the Second Temple Period, but even then it tends to be of peoples they regard as ethnically related and 'fallen astray' rather than people they see as out-and-out foreigners. Ethnic religions are regarded as a pact between the Divine and a specific set of people - and outsiders are either viewed neutrally or negatively coming (or trying to come) into the faith. As such, ethnic religions generally only propagate by birth or, less often, intermarriage. They don't want outsiders flooding into their private ethnic pact with the Divine, and, indeed, sometimes regard it as religiously offensive.

Universalizing religions, on the other hand, are quite explicitly peddling a view of the world that does not, theoretically, have ethnic or cultural borders. Christianity and Islam, both universalizing religions, desire Asian Christians as much as African or European Christians. Theoretically. There is a... great deal of difference between what is taught and what is executed, but in general you can assume that universalizing religions like those more or less always want a person as a convert, regardless of their background. The pact with the divine is regarded as personal, rather than ethnic, and the divine regarded as largely impartial to all the minute divisions of mankind.