Not a heavy pilot but I would have thought that inboard engines in reverse thrust would be more likely to throw stuff into the outboard than vice versa. If there is a structural or aerodynamic reason for inboard only I would think it would have to do with the lateral moments of one outer engine failing to deploy.
Again, as a mere customer I wonder what happens when the tarmac is icy and you are relying on brakes only to stop ' how many tons?'