My theory:
The great depression had ended and the whole country was involved in the war effort and then the post war boom.
During the depression, there was a lot of bartering going on, and money was in short supply due the the devastated economy. My dad told me that, during the depression, you could buy 3 lbs of steak or 3 lbs of hamburger for 25 cents. Unfortunately, most people did not have 25 cents. Losing a dime from your pocket was a dent in the family cash flow. If a kid found a dime or a nickle, you turned it over to mom. As the1930's ended, people's spending habits took a while to change. I was born in 1953 and as a kid, a dime was still a fortune. As late as 1963, we used to comb the ditches for pop bottles to sell at the store. ,
People just held onto coins tighter in the 1930's and so fewer were lost, and many of those were recovered quickly. Everyone walked more and everyone would stop and pick up a found penny.
By the middle 1940's and afterwards, the economy was better and there was more money in circulation. Of course, most of it was coins and 1 and 5 dollar bills. Most vending machines took nickels and dimes. Some did take a quarter, but they would often gyp you so we mostly carried dimes or nickels. It would be hard to estimate the millions of man hours spent by merchants and vendors accepting pennies and nickels for merchandise or to give a dime for the coke/candy machine.
I think that the 1940's, 50's and up to about 1965, people were losing coins about as much as you would expect something so common to be lost. The depression years were time of scarcity and hoarding, silver coins were precious, and you kept up with them.
I also think that after FDR called in all the gold, the people had a tendency to hoard the silver coins, so in the 1930's people would have hoarded gold coins if they could have, but the had to keep eating, so any plans to save gold ended pretty quickly. So people saved silver if they could. By the fifties, a new generation was earning and spending, and the economy was much better. People had "money to burn", even if it was all small bills and coinage. People had more coins, and people lost more coins.
HH,
John Morton