Well technically bolt actions do 2HK with bodyshots, but the 3 hit kill situation is caused these 4 main culprits:
Too agile soldier movement, more precisely a lack of momentum. This makes it almost impossible to lead shots at targets moving in a non-linear pattern.
Soldier can start healing too fast after taking damage.
Bolt action max damage too is low (which is what this thread is mostly about) and the damage drop-off is way too fast.
Bolt action muzzle velocity is too low, this makes it less suitable for attaining high accuracy. Actually, some assault rifles and LMG’s have higher muzzle velocity than bolt actions…