>>3670275The actual answer is because the flange focal distance (distance from the rearmost element to the sensor) is much longer on the SLR-designed A mount than it needs to be for a mirrorless. Sigma made a mirrorless camera with an SLR mount and this is what they had to do.
Canikon would have had a much, much easier time with mirrorless if they could have kept their enormous back catalogue of lenses native, but they decided doing something this goofy was too high a price.
