Because the Shapeshifter will always defeat the Phoenix if they meet on a darker-than-average square. Since the Phoenix starts on a slightly dark square, a standard opening gambit for the Dark is to teleport the Shapeshifter onto the Phoenix, then next move attack the Wizard directly. Unless the Light player moved out a Valkyrie or the Djinni on his first turn, he’s forced to attack the Shapeshifter on a dark square with a Knight or run a serious risk of losing his Wizard before he can Revive the Phoenix.