It's quite simple. The main consideration, which is very important, is the anatomical data on the thickness of seal skulls.
I looked at dozens of young seal skulls in which the bones are so thin that I could break them with my bare hands. In contrast, I would have a much harder time breaking the skull bones of another animal, another carnivorous land animal like a dog or any other animal of approximately the same size. Using a hakapik is truly very effective; it does the job quickly and directly.
Another aspect of the seal hunt that is truly very important and has to be considered, is the extreme and dangerous conditions in which sealers hunt. I was blown away by the skill of the sealers, who hopped from one small ice floe to the next with only a few seconds to spare before the pieces threatened to overturn and throw them into the water where they would ultimately have died within a couple of minutes.
I thought they could use a pistol to skill the animal, which would be far less violent. You would not see any video of big and totally ungainly arm movements. But the danger in carrying a firearm on your belt or in your hand while hopping from one small ice floe to the next would be yet another obstacle for the sealer, who already works in a very hostile environment. What more, the hakapik can help a sealer catch a piece of ice and in the process, get out of fairly dangerous situations.
I think it's a good compromise. You also have to consider the cost of killing. It's a commercial hunt, so if the cost of killing is high, the sealer will not make any profit. In my view, the cost effectiveness ratio of the method employed is truly optimized under such extreme conditions