
It’s because the origins of the holiday go back to the 11th hour of the 11th day of the 11th month when the armistice was signed between the Allies and Germany in 1918 at the end of the First World War.
It was briefly homogenized by Congress into a three day weekend in 1968 but changed back to the 11th day of the 11th month in 1978.
You can find the historical timeline here.