I think there are several separate cognitive abilities needed to ask questions. Curiosity (which is very common), complex communication (much less common), and advanced theory of mind (exists on a spectrum, you need not only awareness of your own mental state, or metacognition, but awareness that others have a mental state that is distinct from your own. Humans actually develop this ability slowly throughout childhood, and it goes through stages). Though there are other species with similar traits, it might well be the case that humans are the only living species in possession of all of them simultaneously.