64 was the last year the were officially minted, except for 40% and proofs, which you can still buy.
Several reasons.
They couldn't bring clad coins on line fast enough because they couldn't get enough explosively bonded clad sheet, and the vending machines needed coins.
The senator from silver producing Montana lobbied very heavily to continue production.
They wanted to punish people who were hoarding 1964 silver coins. (The market price of bags of silver coins did actually drop with the announcement that they were continuing production).
...and in an incredible failure to anticipate reality, they believed this drop would be permanent because despite the fact that the silver value in a coin was well above the face value, no one would feel compelled to keep them because they were so common!!!