Why are some animals seen as acceptable to keep as a pet but not others?

I've had quite a chilled Sunday and was watching videos on YouTube of a woman's pet fox playing with her dogs. All her pets looked healthy and looked after, yet there were comments talking about how cruel it is to keep a wild animal (referring to the fox) as a pet, but NO comments about how it's cruel to keep a dog.

(I would name the youtuber but I forgot her name - sorry!)

What are your thoughts? Do you think keeping a 'wild animal' such as a fox is different to keeping a dog? Also share your thoughts on keeping other uncommon pets, such as what animals are acceptable as pets and what ones are not.