It’s because I usually move my cursor after pressing space, in most software I use the space bar does panning regardless of where the cursor is on the screen and so I prefer that method. When I zoom, my cursor generally doesn’t leave the area that I pressed for the menu.
So for me, it makes more sense to have zoom confined to the middle button and panning to be on the outside. Which is why I would like to customize the space bar menu to move things around since this is mostly a preference thing.