Today there is only one national philatelic society— the American Philatelic Society. But throughout most of the last century there were three national societies— the APS, the Trans Mississippi Philatelic Association, and the Society of Philatelic Americans. Americans were a much more regional country sixty years ago than they are today, and this showed in the choice of philatelic societies. The APS was really more of the stamp society for the North. The Trans Mississippi Philatelic Association was the organization that drew most its support from the mid-west. And the Society of Philatelic Americans— the SPA— was really geographically based in the South which is evidence by its original name— The Southern Philatelic Society— also the SPA acronym.
Before the Depression era boom in philately, which occurred about 1930, these three societies were very regional in base and similar in size. In the post-1930 years two things happened to make the American Philatelic Society— the APS— the winner in the society derby over its two other quasi-national competitors. First, philately does not appeal to all people equally. As the population centers changed in the US in the early part of the last century, the traditional Northern cities grew at greater rates that did the mid-west and the South, mainly because it was northern cities where immigrants tended to concentrate. It was these immigrant groups that in the second and third generations were attracted to philately in greater numbers than Americans had been before. This gave the APS a natural boost in the national philatelic sweepstakes.
But the main reason that the APS thrived was not only because of an innate geographic advantage. The SPA tried to position itself as a national society too (the Trans Mississippi group never did and were happy to be a regional group where everybody knew each other). The APS, beginning about 1940 had a the good fortune to have a very competent and forward-focused succession of officers. They had a very good executive director in Clay Musser and then had the good fortune to have James T. DeVoss as its Executive Director from 1953-1981. Jim was the best— a wonderful, organized, friendly, and gregarious man with enough philatelic chops (he was a revered Canal Zone scholar) and visceral love of the hobby to appeal to all factions. Great Men theories of history don't explain everything, but without DeVoss it is not at all clear that there would only be one national philatelic society today and that it would be the APS.
The Society of Philatelic Americans didn't help itself much either. The SPA had changed its name but not its stripes. Regional in orientation and small minded in operation, SPA conventions were fun for the old boys. But the members of the society forgot that if you can't attract new boys, there end up being no new old boys. Bad management decisions meant huge losses in the SPA sale circuits (there were allegations of outright theft), and in the early 1980s the SPA, after a hundred year history, went out with a barely a whimper. The success of the APS and the failure of the SPA underscores once again the importance of good leadership. The APS has been very fortunate in most of its Presidents and Boards of Directors (including and especially the current President and Board who have done an exceptional job in difficult times). And the history of those stamp societies that have made it and those that haven't show just how critical that is.