I suppose it's possible that different areas and years saw circulation, and that the fad of keeping them wore off and they got spent for a while. My accumulation from circulation was approx. 1974-1977, and then in early 1983 I was completely broke and spent my substantial stack of non-silver Kennedy's on cigarettes (so those circulated again). My late mother-in-law worked for the US embassy in Bangkok and gave me her stack of Kennedy halves she had obtained from circulation in Thailand - range of dates 1964-1976 or early 1980s maybe. Back on topic - it's really pathetic that the US mint has turned this into yet another manufactured "rarity." Everyone in this country should be getting these 250th designs in their change.
My favorite ... the Mayflower Quarter and Liberty Bell, the dime reverse, Independence Hall Liberty Bell / Independence Hall is on a future trip of mine to Philadelphia.