Not that I am old enough to have lived through this but I would guess half dollars fell out of popularity starting from debasement of the metal. The reason is people wanted to hoard the one year only 90% silver Kennedy specimens. When the 40% silver coins were minted people were aware that it was the only general circulation piece left that contained any silver, so they kept hoarding them even though these coins only contained 40% silver from 1965-1970. Perhaps by the time 1971 came around it was no longer worth it to carry these large monstrous half dollars since they didn't buy enough to make them worth the effort or headache of carrying around. In 1971, it cost roughly 35-40 cents for a slice of pizza. So when the buying power of large coins fade and they make holes in your pockets, cause change pursues to burst and not close, so does their circulation and popularity fade (especially when they contain valuable metal like silver).

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