How about LHR - YYZ? Daily I see a 747 and a 777 (BA) in Toronto. Why not one 380?
Possibly not enough premium class traffic. Toronto is a very Y-class "visiting friends and relations" destination. It's one of the few North American points that still runs to charters from the UK. Would be a bit wasteful to have most of the A380 upper deck taken with op-upgrades.