You ask a good question. While it's true that Judaism did spread into Europe, that doesn't make them of the Jewish bloodline. It's often believed that the Jewish bloodline passes through the mother, but that's not true. The Jewish bloodline passes through the father. That's why the Generations talked about in the Bible are separated by the male figure and not the female. And that's why you have a "Land of Israel" and not a "Land of Miriam".
Also, true Jewish blood is made up of those Jews who do not intermarry. The bloodline has to remain pure and not mixed with Gentile blood. While these mixed-blood Jews are still considered "Jews" and descendant from the Jewish lineage, they are not Jews that are in line to inherit the Promiseland.
True Jews are Hebrew and have olive-colored skin. Anyone can convert to Judaism, just like anyone can be a Zionist. Only the unbroken bloodline, however, designates the true Jews.