Because one is measured by what's in his/her heart. By acting passively, with greater love, understanding, patience and tolerance we strive to be closer to the LORD and his guiding principles. 

Hostility, anger and retaliation are all feelings that stem from an evil thought or origin. JESUS doesn't want us to be robbed of the joy the LORD blesses us with daily. This is why he led by example and taught us in the following manner. 

Jesus instructed his followers to react passively when attacked as to show we follow His teachings with love and kindness and let God decide someone's punishment, not ourselves.  Blessings.