I have my own theories. As has been mentioned, it's the border between familiar and wild. Most people have had a dog at some point in their lives, and it's common knowledge that the first dogs were domesticated from wolves. So if you wanted to go for something familiar, but still wanted that tie in to the wild, wolf is it. But what I think is the bigger factor is that wolves are some of the most diverse animals when it comes to behavior. Everyone knows about the strength of the pack, but the term lone wolf also floats, so as an animal, it can apply to either type of individual. Wolves are loyal to the ones they know, but distrustful to others. A common human trait. Wolves are playful when young, but appear more serious when they're older. They are beautiful, but tough and cool too. They run the whole range of personalities, just like humans, so it is easy to see ourselves mirrored in them. Plus wolves have all of the reasons people become furries covered. People go furry to become part of a close-knit community. Wolves form packs with others in similar fashion. Or furries join to instill some self confidence in themselves. Wolves appear fearless in most of their media portrayals, as well as in the wild. Some people are in it for the aesthetic appeal, and real wolves have a lot of natural beauty. Maybe some are just in it to get noticed, and wolves are one of the most popular wild animals. No other animal hits all of these different reasons so effectively. Some hit individual reasons better, but none manage to hit all of them so well. Anyway, those are my theories. Sorry for the long post.