Here's why:
* Soul Exchange's effect: Soul Exchange requires you to select 1 face-up monster you control and 1 face-up monster your opponent controls.
* Face-down monsters are hidden: Face-down monsters are hidden, and their information is not available to either player.
* Card effects need clear targets: For a card effect to work, it needs to have a clear target that is known to both players. You cannot select a face-down monster as a target because you don't know what it is.
In short, you cannot use Soul Exchange on a face-down monster because it requires a face-up monster as a target.