Not the aicrafts fault
Like its 'bomber' counterpart the Fairey Battle, the Defiant had been ordered to suit the requirement of the day. (several years earlier).
The Battle had replaced the likes of the Hart with no contemplation of the German flack or fighters. and the Defiant was in the same mould designed to take out Bombers (which did not employ escorts then).
Both performed as designed, unfortunately the enemy had changed the rules by then.