My reason is very simple.
Baseball cards are meant to be held, to be handled. I love having a stack of cards in my hand and then thumbing through the stack. And it seems that a stack of vertical cards fit perfectly in any sized hand. They fit in my hand when I was eight years old. They fit now.
Horizontal cards don't fit in you hand the same way. It is very awkward to thumb through a stack of horizontal cards.
In the series one Cubs cards there are three of these horizontal abominations.
They must be changed.
A quick trip to Getty Images got me the original photos use by Topps. A little more playing around and, voila!