Was this the only reason or one that is the most convenient to cite?
The original reason was the Jewish religion. It was the first moralistic monotheism in which the same moral rules applied to both the rich and the poor, young and old, workers and kings. Everyone had to follow the God-given laws and the fact that you were strong, powerful and rich didn't give you the right to oppress and exploit the weak, insignificant and poor. Imagine how this was received by the rich and powerful of Antiquity :) Judaism was the ultimate heresy for every absolute ruler of Greece, Rome, Persia, and for the ruling elites of any ancient kingdom.
Then, because of the said hatred, Jews were barred from entering many professions, from taking positions of power, certain offices etc., so they took upon something that was allowed for them but generally frowned upon for religious reason - money lending (back then known as usury). So, it added brand new hatred to the old religious one, reinforced by the general lack of education and idiotic legends e.g. about ritual murders spread by haters throughout ages.
The fact that Judaism was never really a "conquering religion", like Islam or Christianity, and that potential converts were rather discouraged than encouraged to join - which created closed, hermetic, communities - didn't help either.