One why .. Racing Cyclocross , and dismounting on the left side , you can be half way off the bike
and slow down with the rear lever on the near side if you are coming up too fast to a barrier leap.


I had another Why , on my Pushing my loaded touring-bike up a really steep road, also dismounted on the left, I,
was grabbing the near side brake lever to catch my breath, and let my heart-rate drop.

and the bike was not rolling back down the hill, dragging a skidding stopped front wheel, behind it.

3rd why, -because it came that way and I saw no reason to change it ,
so not exclusive One way or the other ..