because the 380 is a monstrosity that requires entirely new infrastructure to operate. it also requires insanely huge loads.
the 777 and 787 fits way more into AC's model; fly anywhere point to point, not 500 heads of cacttle from yyz to lhr. this allows more choice for travellers; ac can go YYZ-BOM, YYZ-MEL etc. the 380 only means YYZ-LHR, YVR-FRA and limits choices in schedules.