Just look at the ads in Broadcasting from the 30's and well into the 40's. Stations showed mail pull, maps and all kinds of evidence of listening in many states.When did the light finally start turning on in the heads of the nation's radio station owners that there was no bottom-line value in pumping a 50,000-watt signal over 30 states? Did DXers actually figure in business decisions before then? And why? Did advertisers back then actually want to reach listeners hundreds of miles away, or were station owners just assuming they did?
Even a few regional channel stations like KMA and WNAX ran ads showing extensive coverage of broad agricultural regions.
They ran those ads because much of America had no local night service, and some of those stations had star-power shows. The country music on stations in Shreveport, Wheeling, Nashville and the like got huge audiences and had waiting lists for advertisers.