King Duryodhana also could never be killed in a fair fight! The same is the case with all those powerful car-warriors led led by Bhishma.
नैष शक्यः कदाचित् तु हन्तु धर्मेण पार्थिवः। ते वा भीष्ममुखाः सर्वे महेष्वासा महारथाः॥