It's more natural for your palms to be basically facing each other. That's how they are when you're just standing up. Also think of steering wheel, joystick on e.g. a crane, or control yokes on aircraft where people have to hold it for hours at a time and possibly apply force. Neutral grip is also a thing in resistance training, for instance neutral grip pull-ups are easier on the joints. It stands to reason the sideways mouse, and some kind of keyboard that allows the hands to lie in a more natural position, would be less stressful on the various small parts. You don't want to be constantly moving things far away from their natural position!