DavidKaye said:
Since the late 1920s the only people interested in DX were elderly men and 13 year old boys, and fairly anti-social ones at that. That's it. Nobody else was interested. This is why radio stations, even those clear channel monsters like KGO and KNBR don't try to sell their out-of-market audience. It's not significant.
In sort of reverse order....
KGO and KNBR don't sell out of market audience because there is no one to buy it.
In the 20's and 30's and much of the 40's, there were few stations (around 800 on the air at the close of the 30's) and half the listeners in America were DXers.
Take Traverse City, MI. The market, in 1940, had no stations. In 1960, it had one with an FM simulcast. Today, the market has 42 stations. In the 40's, folks listened to WBBM and WGN and WJR and WLS... and WSM and WCCO and many others. In other words, everyone DXed. And if you lived in most of the counties surrounding Traverse City, you had no listenable night signal short of Detroit and Chicago.
Through the early 50's, stations advertised in Broadcasting and Sponsor and Radio Daily talking about mail pull from dozens of states and hundreds of counties. WLW was "the Nation's Station" well into the 40's, even after dropping power by 90%.
Somewhere in the 50's (and not the 20's) agencies started to buy local markets, following the TV model... since TV did not have added night skywave coverage.
And, also in the 50's, radio's prime time changed from nights to daytime due, again, to TV. The huge nighttime shows and network extravaganzas were swept away by video.
DXers, who listened to distant stations not just for entertainment but for the sport or challenge, were far too numerous to count well into the 60's. While hard core DXers with Hammulunds and Nationals and such were far fewer, nearly everyone got a thrill out of hearing a baseball game from St Louis or a barn dance from Wheeling... and many discovered it was fun to hear distant voices. Since the bands were relatively clear, logging a 250 watt graveyarder from California in Ohio was possible even in the 60's... and teens in Raton NM heard the latest songs from KOMA and those in Caribou, ME heard them on WKBW.
DXing severely declined going into the 70s because the channels were so congested and noise levels increased.
Oh, and I was an AM DXer... a 13 year old one... 2300 stations, 87 countries, all states and provinces... and not terribly anti-social; I edited my school paper, worked part time at a radio station, did record hops, ran on the track team and wrote poems for poetry magazines and called stations in Mexico to request songs. DXing opened my world... on April 15th, 1961 I played for the school current affairs club a recording of Radio Americas from Swan Island encouraging all of Cuba to join in the Bay of Pigs invasion and to free themselves from communism... a recording I had made the night before!