Aggregation. Aggregate services. Guy who made padmapper.com just aggregated all listings from various sites.
People are visual and they know maps, but seeing lists is harder to parse.
The fact that nobody else at that time had put apartment search on a map is a great example of how low the bar is for excellence in this space (no offense, @padmapper).
I'm pretty sure that housingmaps did it earlier. However, padmapper provided a much nicer experience.