Robertson in Canada is just appeal to tradition at this point. They have no technical merit over Torx except maybe ease of ad-hoc construction of a driver.

Robertson are generally cheaper than Torx, for one thing.

Robertson has a really nice taper that makes one handed operation really easy even when the screw metal isn't magnetic. The large size of the recess also means you can work Robertson screws even after they have been painted or varnished over. Robertson sizing is also incredibly simple so you grab the right tool every time. Each size has a colour name so you can easily call out each size to a tool fetching assistant (always my role as a kid) and there is no ambiguity if the screwdriver is the right size or not.

Not a ton of advantages over Torx for initial assembly, but if you ever need to repair, especially someone else's work, Robertson is second to none.

Not rare at all in the US, in my experience. I've been doing some construction and tear-down work recently, and it's about 60% Torx, 40% Robertson.