The simple reason for this part of the creative process is that our eyes are set side-by-side in our heads, making the landscapeview more pleasing and open than the portrait view.
For your presentation development, you can do your brainstorming on a whiteboard, a computer screen or Post-it Notes as you generate your ideas, but what is as important as the free flow is that you see the ideas you generate in a panoramic or landscapeview.