You have to look at it in the context of how Europeans looked at Jews over the millennia. The ordinary Jews were usually highly segregated in the ghettos or villages, with the well-off families sometimes allowed to live among the general population. But the deciding line was always religion, especially since you couldn't place a distinct nationality around it, based on any normal notion of nationality. Thus, by default by the time middle ages, Jews acquired a nationality by virtue of being of a Jewish religion, and not coming from a particular nation or region, as they were removed from their native land by some 500 years.