The country was in the grips of a severe coin shortage. Much of the cause was the increasing value of silver and the rapidly growing vending industry that had coins tied up in increasing numbers of vending machines.
But collector activity was very high profile because there were ads to buy and sell bags of brand new coins. The modern segment of the coin market was on fire and recent date bags and rolls had huge premiums because everyone figured the demand would grow to the sky. A common '50-D nickel bag went for a quarter million dollars in today's money!
The mint and Congress saw this and quickly blamed collectors for the shortage despite the fact much of the shortage was in silver which collectors were not hording in large quantities. There was even a bill before Congress (Bible bill) that would have effectively banned collecting all modern
US coins. But they weren't done punihing collectors here; They ended production of both mint and proof sets. The SMS sets weren't announced until months later and it was years before the mint and proof sets would resume. They instituted a date freeze in 1964 that would assure all coins until the end of time carried the 1964 date. Even when the switch was made to the clad material that collectors hated so much they announced that the 1965 date would be frozen indefinitely.
As if all this weren't enough to discourage anyone from ever collecting modern coins again, they also eliminated mint marks. Coins were being made at all three mint for circulation but none carried mint marks. 1964 dimes were still being made as late as Feb, 1966.
The modern coin market simply collapsed in '64/'65. Even today you can pick up many of these bags for face value or little more. The highly touted '60-D small date was the most common of all penny rolls until 2006.
The mnts have been striking lots of coins at the San Francisco and West Point mints for circulation and without mint marks since 1967. Usually these are smaller mint runs and they fear that collectors would save them if they were identifiable.
Time don't fly, it bounds and leaps.