It would indeed be interesting to learn if there was a time or event that "killed" radio for Walmart, or did it never get started.
If you are a "disciple" of that Midwestern broadcaster of a generation ago, Jerrell Shepherd, then you assume that success in radio advertising for the retail merchant is built on the foundation of "price and item" commercials, or as I think they phrase it today: Transactional copy vs. image copy. Print advertising, particularly in small towns were Walmart took off has always been highly 'transactional copy' so maybe there was no great emotional event in the life of Sam Walton. Maybe for him print worked and voice didn't.
But getting back to the original post that kicked off this thread: I had a whirlwind career in broadcasting for a few years and they got out for a number of years. When I traveled for business reasons and when I traveled for personal reasons, I still followed radio as I went. Spend a day or two in a town and I'm all over it: listening, checking out listings in the phone book, watching for billboards, paying attention to radios being played in small restaurants, and in some cases when I had time, doing a drive-by to size up the facility of a station.
I go back to the premise of the original post. Radio does not invite people in. Radio invites it's regular listeners to come in and participate... but radio HIDES from people who are not already part of the listening family.
I drive past transmitter facilities that do not have a sign out front to identify who they are.
I was in a small town a few years ago that had a station. From a distance I listened to it from time to time. Sometimes they would do a remote broadcast from the town where I lived. (You would only know that if you listened to them.) I stopped for coffee at a convenience store and caught a local policeman in the parking lot: "Where is the radio station?" His response: "Mister, I grew up in this town. There ain't no radio station in this town."
I came back a couple of days later after going to radio-locator and I found the station about two miles from where I had the conversation with the policeman. That, my friends, is a picture of radio today. As long as our salesman can sell SOME advertising, we don't care if anybody knows who we are or where we are.