It has been 30 years since I last took a tour of a sawmill. Back then all the bandsaws were vertical. The only reason I can think of is that it would not be a good idea to have several tons of log fall down and pinch a horizontal blade. I suppose it also makes installing the blade easier since you can lift at only one point not both ends at once. Of course as soon as a slab is cut it is flipped flat and sent on it's way.
It still surprises me to see a logging truck with more then one log. It just seems like what is the point to be hauling such small stuff. Of course intellectually I know all the old growth is gone.
Bill D