I think part of it is that Erdos worked with an unusually large number of people, at an unusually large number of places.  Another reason is that he was quite eccentric, even by the standard of mathematicians.  There are so many stories about him, which led to a sort of legend, and that led to all the books and documentaries.  Finally he was around at a time when discrete math was gaining traction as a de facto area of mathematics, and he made a great number of contributions in that area.  For example the probabilistic method he popularized is a cornerstone of the subject nowadays.<p>Whether or not he was <i>disproportionately</i> popular depends on what you feel it should be proportionate to.  I reckon he is more famous than similarly important figures in other subfields of math.  But few mathematicians have worked with and influenced so many people.