Total spitball here, I base this on nothing.
Dogs are typically cast in media as always good, cats always bad, while foxes are sly and a kind of intermediary between the two. Ambiguous enough for kids growing up to read into them however they want and imagine themselves as the character with more freedom of choice.
Visually, foxes are a bright orange/red mix that stands out, they kind of look like a mix of cat/dog/wolf. Kids grow up used to cats & dogs as pets, wolves are too dangerous to consider, but foxes are in-between the two. Threats to smaller animals but never presenting a threat to people. Furry artists use this themselves, an interplay of good/evil like the picture you responded to. It's used in Zootopia, Fox and the Hound, the furry Robin Hood, I'm sure there's others, but all the characters are capable of both violence and having good hearts underneath. That's appealing to teenagers and young adults.