I didn't look into what the provinces have listed as their statutory holidays, but I assume that in each province they have passed it as a statutory holiday. If I'm wrong on that, I stand to be corrected. The provinces have the constitutional jurisdiction to regulate days off for regular working folks, schools, and all of that. That's in their jurisdiction.
I assume July 1 is considered a statutory holiday because at some point a law was passed in each province deeming it so. Four provinces don't have Victoria Day, and two don't have Remembrance Day. There are two that do it differently.