semoochie said:"(Otherwise, how do you explain to listeners why you are playing 3 Dog Night's 1973 hits but not their 1969 hits?)" It could possibly be the same way that Elvis Presley's 100+ catalog was reduced to two songs! They quit playing the Beach Boys 1963 hits and everything by the Four Seasons before the late 70s as well!
Funny, though, that two of the Seasons/Valli songs still on many classic hits stations' playlists are "Grease" and "December 1963 (Oh What a Night)," sentimental songs about years the stations can't play songs from!