Cannister has been a standard artillery load at least since the Napoleonic Wars.
All of our artillery has it, and all our tanks did through the 105mm gun on the M-60.
At least during the Vietnam era there was also a Flechette Round which was a cannister filled essentially with 20 pounds of nails.
I can't imagine why it was never provided for the 120mm.
So9
That's easy...the M1 Tank was conceived first and last as a killer of other tanks (unlike earlier US tanks). That is why they gave it the gas turbine engine, the night vision systems, stabilized gun, and eventually that 120-mm Rheinmetal smoothbore.
We had a 90 MM canister round for the main gun on our M48A3 tanks in Vietnam. Worked damn good against RPG gunners in ambushes, and other living things as well.