Trade & Survivorship: One of the interesting trends you notice reading Roger's book is that survivorship is inextricably linked to international trade (mintage also matters, but not as much as you might think).
In years where international trade takes off, you have more gold shipped overseas, especially to South/Central American and Europe. These coins were held by individuals, institutions, and SDBs. But they escaped the gold melting of the 1930's.
Where trade dried up, you kept more of your domestic gold production and less was shipped overseas. Of course, the extreme case was during WW I when there was virtually no trade and from 1917-1919 no Double Eagles were minted. Another clear connection is the small numbers of the "Fab Fives" (1928-32 Saints) that were shipped overseas following the stock market crash and the onset of the Great Depression, Smoot-Hawley trade barriers, global currency debasements, etc.
The Roarin' Twenties saw the U.S. escape the recession of 1920-21 and the rest of the world escape the carnage of WW I so economic activity was good.....plenty of SG Double Eagles sent overseas or south of the border and these issues remain very plentiful with a couple of exceptions.
The correlation isn't perfect as it doesn't hold as much for earlier years (i.e., 1907-1908 during The Panic of 1907). But international trade increased alot more in 20 years after WW I for a number of reasons (early globalization, rebuilding of Europe, Dawes Plan, etc.).