In Canada, Remembrance Day started as Armistice Day in 1921. On 23 May 1921, the Government of Canada passed the Armistice Day bill which stated that the Monday of the week of 11 November would be called Armistice Day to commemorate the Armistice and the First World War; Thanksgiving was also celebrated on the same day. In June 1931, Thanksgiving and Armistice Day were split after lobbying from a variety of groups (e.g. veterans organizations, women's groups, politicians) to separate the holidays*. This legislation also changed the name from Armistice Day to Remembrance Day and set the date of the holiday as 11 November. The name change was partly done to broaden the idea behind the holiday from one which focused on victory to one which reflected commemoration more generally.
The choice of 11 November as a day of remembrance stemmed from the armistice signed on 11 November 1918, ending hostilities on the Western Front. On 11 November 1919, King George V requested all British subjects (which included Canadians at the time) observe the anniversary of the signing of the armistice by two minutes of silence at 11:00 AM.
What did Armistice/Remembrance Day represent? As Vance argues, it affirmed the dominant myth of the war in Canada - namely, that the war was a unifying and nation-building experience for Canada. While Armistice/Remembrance Day was intended to commemorate the sacrifices of Canadians, particularly those who had lost their lives, it was also a means of reinforcing positive ideas about the war as a worthwhile conflict which defended high ideals such as liberty and democracy. As Vance writes:
As it evolved through the interwar years, Armistice Day became an affirmation of the myth of the war. The eleventh of November was a day to pay tribute to the men and women who laid down their lives in a just cause and to reflect upon the nobility of the sacrifices made for Christianity and Western civilization. It was not an occasion to wonder if Canadian soldiers had died needlessly or to recall that many of them had suffered dreadfully in the trenches. It may have been a day to pray for peace, but it was not a day for pacifism. On the contrary, it was a day to honour those people who had been willing to fight for peace. (Vance, Death So Noble, 219)
As is the case with public memory generally, public commemorations reflect the ideals and needs of a society. Immediately after the war, these included honouring those who had died and asserting that their deaths were meaningful.
So why did a holiday like this develop after the First World War and not an earlier conflict? For one, there weren't many prior wars which Canada could look to. That doesn't mean that previous conflicts weren't commemorated though (e.g. commemoration of the Battle of Lundy's Lane in 1914). The First World War, however, was a war of mass mobilization unlike previous ones; it affected Canadians from coast to coast in one way or another (e.g. military service, war work, bereavement). As well, the First World War caused significant loss of life. Out of a population of almost 8 million, approximately 600,000 served in the Canadian Expeditionary Force and over 60,000 died. In comparison, Canada's previous conflict, the South African War (1899-1902), had just over 7,000 Canadians serve and less than 300 dead. In particular, it was this mass loss of life shared across the country during the First World War which inspired Canadians to commemorate the conflict, such as through Armistice/Remembrance Day but also memorials, poetry and artwork, to assuage their grief and affirm the positive impacts of the war.
* "Holiday" isn't necessarily the correct term for Armistice/Remembrance Day as its status as a holiday is/was debated and it is currently not a statutory holiday in every province but I've used the term for ease of reference.
Sources/Recommended Readings:
Jonathan F. Vance, Death So Noble: Memory, Meaning and the First World War (Vancouver: UBC Press, 1997), particularly Ch. 4 "If Ye Break Faith"
Tim Cook, Vimy: The Battle and the Legend (Toronto: Penguin, 2018)
I am not a British historian but I recommend Adrian Gregory's The Silence of Memory: Armistice Day, 1919-1946 for a British history of Armistice Day.