My own guess - there’s already a bias towards right handedness. I assume that is related. I don’t know how these staircases were built but it may be that it’s s

My own guess - there’s already a bias towards right handedness. I assume that is related. I don’t know how these staircases were built but it may be that it’s slightly easier to build these staircases for right-handed people. And then maybe 30% of builders were left handed or some minority of the time the architect wanted to prioritize symmetry with another tower.