Because grey is the ultimate neutral and the garish ads will stand out against it better than if the building were scarlet, or purple, or orange, or emerald, or baby blue, or shocking pink ...
True, but even if some elements were coloured. Especially on the sides with no/less ads. Some sort of contrast on those, things, sticking out near the top or on some of the criss-cross panel work.
At least something's going in those two spots though, hopefully Ryerson makes it interesting.
With that opening onto the old parking lot, it looks like the whole building landed on top.
A parking assimilated by the Borg Mall.