I think it had more to do with the author's ability to portray the inner workings of White Fang as driven by instinct and possessed of a child-like innocence. The scene where he is sold and his mother is taken away with her owners is very sad and desperate. Maybe because the author conveyed no drive for vengeance that might occupy a human protagonist.

I'm not sure about the other books you read, but maybe you are so affected by them because they dealt with a state of nature - there are no tea parties, broken dates and afternoons filled with sighing. The animal protagonists deal directly with survival at all times. Anything that distracts from that is often mercilessly weeded out.

Or perhaps it's the sense that these beings are so similar yet different from human beings, and there can never be a way to completely bridge that divide. Even with a beloved pet, there is no way to know what they are thinking at any given moment. Sadder still, this holds true for the vast majority of people, which is why we often hold on most vehemently to those that can complete our sentences and follow the tracks of our thoughts with no prompting. Reading an animal perspective often reminds me that divisions do exist an we are often cast in the role of enemies because we are different.