In ancient times, it usually happened that either one army enveloped the other, or it completely blocked its path of retreat by attacking its rear or flank after its own cavalry defeated the enemy cavalry. Thus, if the surrounded side lost, it was completely cut down, as happened at Cannae.
Given that in medieval times the difference between combat branches was deeply blurred, that there was no actual cavalry or infantry, the battle consisted of a direct, frontal clash between the whole of the opposing forces, the path of retreat being clear. In case of defeat, the losers fled, whilst the winning commander lacked the effective authority to order his troops to pursue the enemy.
Moreover, the men fought not in order to kill the enemy, but in order to capture him alive, in the hope of obtaining a handsome ransom. This inclination, Hans Delbruck notes, weakens substantially the warrior spirit, which must stay put in annihilating the foe. The same renowned military historian notes that he was yet to find an instance of medieval battle where one side purposefully detached a squadron so that it may land a surprise attack after the battle started. This is the only imaginable case where an army's retreat path is blocked, thus leading to it's total annihilation. In the absence of such scenario, medieval battles were essentially limited in bloodshed.