You chop them lengthwise to make it easy to pack them out on stock saddles. They pack easier in rough country or long distances, than chopped crossways.....
Lower center of gravity, less easy to hang up in brush/trees and harder to buck off the load than crossways, too.
Now stocksaddles or packsaddles with panniers are different story, but it is still easier and WAY faster to chop lengthways and then crossways if you bring them out in bone-in/skin-on, in full quarters.
