I have a theory - nothing more, no facts to bolster my case - but the theory says that Casey Kasem started the American Top 40 in 1970. The show was very successful and was in just about every market, usually on a CHR station, sometimes the biggest CHR station in a market all through the 70s and early 80s. Thus the American Top 40 homogenized radio across the country and it was from that point forward that you began to see a lot less regional or local hits.
I can see why you think that, and it makes logical sense, but AT40's impact in any market was pretty minimal.
While
everyone (it seems) heard American Top 40
sometime, it wasn't a show that entire cities listened to
all the time. It usually aired in non-prime weekend time slots. Three or four hours out of a 168-hour week. And most of the audience wasn't tuned in for the whole show.
Actually, AT40 played records that weren't getting on the air in a lot of its markets...lemme grab a Whitburn Pop Singles Annual and look at the songs that peaked at #40 during the first half of the 70s, after AT40's launch on July 4, 1970:
Marvin Gaye: The End Of Our Road
Delfonics: Trying To Make A Fool Of Me
Joe Simon: Your Turn To Cry
Yes: Your Move
Ten Years After: I'd Love To Change the World
Four Tops: Just Seven Numbers
Ashton, Gardner and Dyke: Resurrection Shuffle
Fanny: Charity Ball
B.B. King: Ask Me No Questions
Barbra Streisand: Where You Lead
Tommy James: I'm Comin' Home
Beverly Bremers: We're Free
James Brown: King Heroin
Jerry Lee Lewis: Me and Bobby McGee
Elvis Presley: Until It's Time For You To Go
Rod Stewart: Angel
Luther Ingram: I'll Be Your Shelter
Temptations: The Plastic Man
Gunhill Road: Back When My Hair Was Short
El Chicano: Tell Her She's Lovely
Bette Midler: Friends
Curtis Mayfield: Kung Fu
Barry White: I'll Do For You Anything You Want Me To
B.T. Express: Give It What You Got
Marie Osmond: Who's Sorry Now
C.W. McCall: Wolf Creek Pass
Temptations: Happy People
Doobie Brothers: Sweet Maxine
Especially in markets with tight playlists, most weeks, the bottom 20 of American Top 40 was actually
adding diversity to what was airing there.
Also, in Los Angeles, Billboard was often weeks behind KHJ and KRLA, which left AT40 with records that were peaking on the Hot 100 that had already burned in L.A. Let's take Don McLean's "American Pie" as an example:
"American Pie" hits #1 in Billboard 25 days after it hits #1 at KHJ---and 11 days after it starts moving back
down the chart at KHJ. At this point, KHJ's been playing it for ten weeks. Casey's only been playing it for four.
A month later, "American Pie" is still number one in Billboard and on AT40. It fell
off the KHJ playlist that week (February 8).
It's still in AT40's top ten the last week of February. It's still in Casey's top 20 the next to the last week of March, and two weeks after that, he can finally get off it because it drops 22-56.
The biggest factor in the nationwide homogenization of the hits was Bill Drake leaving RKO. As David notes above, Drake (especially in the Ron Jacobs years, but also after) believed in turntable records. His successor, Paul Drew, didn't. Under Paul, it was immediately boiled down to what was selling.
And that affected New York (WXLO), Los Angeles (KHJ), Chicago (WFYR), San Francisco (KFRC), Boston (WRKO) and Memphis (WHBQ). Add that to WABC, which ran between 17 and 22 currents, the "Q" format stations, which tended to play 22-25 and the stations Paul was consulting before he got the RKO gig, and most of the influential Top 40 stations in the country were, by mid-1973, waiting for records to become undeniable hits rather than trying to break them in their markets.
Also, I think what happened at a similar point in time is the record labels got really successful in the commodification of their product and began to sell it on a much more consistent, national basis than they had previously. By 1980, the truly local hits had become very few. The music market was a completely national one.
And yes, absolutely. That pretty well killed off the markets and stations that were still taking chances and breaking records. The only exceptions I can think of past that were KROQ under Rick Carroll (spawned a few CHR hits), and Guy Zapoleon at KRQ in Tucson and KZZP in Phoenix.