I don't understand why you'd use flat roofs in canada, doesn't it rain and snow a ton? That sounds like recipe for leaking and collapsing roofs compared to angled ones.
Incidentally, mono-pitched roofs seem oddly rare, people seem to go straight from flat to gabled or hipped, even in relatively temperate places where snow doesn't pile up much and a relatively shallow slope would be more than enough to handle rain and snow.
Kinda late, but the reason is building height restrictions. It lets you squeeze in an additional level in urban areas like central Toronto. Plus some people like the modern look I guess.
Incidentally, mono-pitched roofs seem oddly rare, people seem to go straight from flat to gabled or hipped, even in relatively temperate places where snow doesn't pile up much and a relatively shallow slope would be more than enough to handle rain and snow.