The Canadian flag is older than I am. And like Hermann said, it's immediately recognizable. The other designs on there are variants on other flag design, and take that moment to figure out what's different, which flag it is. For someone with say, a military background, that might be a snap, but, average Joe Blow on the street, not so much.
In today's global economy, 'branding' your country for export is almost as important as keeping your citizens happy. And Canada is branded. It's recognizable. It's distinctive.