because the Spartans were fierce warriors and were probably the strongest and toughest men of all time, since they fight since they're childhood. They never give up, because for them it's an honor for them to die in battle. The Spartans traitor did'nt really do anything, he just brought the fight, but he didn't cause them to lose. Xerxes army, if it was evenly matched, would've been demolished, since Spartans kill wolve as children, and fight in war since they're teen years. They were simply outnumbered
