Interesting, interesting, interesting note.
Look at Chris Lash's list, then look at Oldies Cat's list. (n.b., Chris--perhaps intentionally (see, e.g., speculation over at the Steel City board)--left off 3WS, WWSW-FM, Pittsburgh).
Tell me (a) where the delineation line is drawn (note, it sort of rhymes with Base 'n Mixin'), and (b) what the population movement has been in those areas.
(Generally--there are, of course, exceptions.)
The markets with NO FM oldies station are concentrated in the major urban centers (NYC, SF), the South, and the West. It just so happens that those areas are (a) the prime locations for new corporate headquarters and other business, and (b) the largest population growth areas in the country.
The markets with FM oldies stations are, again, for the most part, north and east of the Mississippi. In other words, the old population center, Rust Belt, colder climes, and older population, overall.
It all comes down to, again, knowing your audience and knowing your market. If you have an influx of younger people flooding to New York, you're not going to waste a full signal on oldies that could otherwise program Top 40, or AC, or an ethnic-targetted version of those.
But if you're in Cleveland, and (as John Gorman once intelligently told me) you're older and still living in Cleveland, chances are extremely good you were born there. Thus, you have an older population, not as much pull from the younger folks needing to be served (there aren't that many there, and there comes a point where there are diminishing returns of too many focused), so oldies stations have more staying power.
It's simple concentration of demographics--let's say you have potentially 100 oldies listeners. 85 remain in Cleveland, for example. The other 15 have dispersed to all of those numerous better climes south and west (two to Vegas, three to Atlanta, two to Charlotte, three to NYC, one to Frisco, two to Austin, one to Tampa, one to Memphis).
What remains is a group of people who may or may not like oldies, but who, if you are programming to them, are not needing a vanilla Top 300 playlist, like K-Earth specialized in. You can target those 85% who stayed in Cleveland from childhood with more local flavor, more remembered songs from then and there, instead of just then.
Worked for John Gorman.