People found county late at night when the radio "skip" occurred from stations like WLAC and WSM out of Nashville, and others..
I don’t know when country became more popular nationwide.
I do recall, living in Chicago in the early 1980s, that WMAQ was a major country music radio station. So at least by then, country music was popular in some big cities.
The only real skip I listened to was WKBW in Buffalo (Rock/Top 40). Although during BCT and AIT at Fort Knox I could pick up WBZ in Boston. And IIRC I also picked up WBZ in Florida as well.
Back in the day before Satellite Radio there was 820 WBAP in Fort Worth to keep the truckers up during the night.