Because the barrel is not rigidly attached to the receiver like it is in a bolt action.

The shell is chambered inside the barrel and the bullet travels down the barrel not the receiver.

It is, 99% of the time, benneficial, accuracy wise, to mount the scope to the barrel in guns with removeable barrels.

If you were to never remove the barrel from the receiver then either one would be just fine. But in order to take a shot gun apart and clean it well you must remove the barrel.

Another reason is that should this become a dual purpose gun (slug gun and bird gun) the scope does not have to be removed and resighted every time he switches from deer to birds. The scope stays firmly attached to the slug barrel thus holding its zero pretty well. At most he would have to take a couple of verifying shots and a click or two. Versus remounting the scope and resighting every single time he changed.