I think his location has a good bit to do with it.
St. Louis/E. St. Louis was a very active and vibrant african-american city in the 50s-70s. It was a northern city in that the population had money and could afford to buy records. Similar to Chicago, Detroit, Cleveland, etc. (Contrast this with some southern cities like Atlana where I now live.) A lot of the stuff we like to collect was sold there.
Lastly, I might be that when the Japanese collectors hit the US hard (I wasn't collecting records back then so help me here) and cleaned out alot of places, may be St. Louis was passed over?
Couple these demographics with a smart, active collector with a nose for buying up collections over the years, and that probably explains it.