Is it completely arbitrary or is there a reason? Is it maybe because it is easy to remember? I feel like there has to be a reason but I am not finding anything with an answer... Frustrating!
Are you asking why Armistice Day is celebrated at that point today? That's a very easy question - the World War I Armistice took effect at 11am on November 11. Or are you asking why it took effect at that time on that day back then?