The reason, I guess, is that if it were not so, the King could position itself next to the centre square and never be captured, even if it couldn't escape that way as the requirement is to place pieces on all 4 sides.
(gem) Du kan bruge nogle af de simple HTML-koder i dine beskeder eller hvis du er et betalende medlem kan du bruge Rich Text Editor. (pauloaguia) (vis alle tips)