David, any idea why, during the past week, KRTH would play the Beach Boys' Dance Dance Dance (8/1964) but not Good Vibrations (1/1966)? And why are certain '60s songs, such as Wild Thing and Brown Eyed Girl, played ten to twelve times a week while others are completely ignored? And how much longer before KRTH drops all the '60s music?
As to why one song plays and another song does not, the answer is usually in the test results which answer the "do people want to hear it today?" question.
Stations don't program by decade or era. They program by target age demo. If the target likes a song, they play it.
Keep in mind that Brown Eyed Girl is a rather interesting exception in that it did not get a lot of airplay as a current when compared to other songs... it sort of crept up on the audience over the years and is as close to an anthem as we see in classic hits.