Because "consciousness" at least in the way people colloquially refer to it seems to be dependent on time and spatial awareness (i.e. I am drinking tea in a cafe in Paris, France), and independent from animal instinct. Moreover, since "I am" drinking tea in a cafe -- I would argue that it's a priori that I know exactly what am (e.g. human) and doesn't have to be stated. Thus, in the human experience - for us - concisouness is the aforementioned (i.e. time, place, I am) and most of our methodology in regards to determining what makes a species conscious -- or not is built upon those notions (like an animal being able to recognize itself in a mirror).
Yet, that's a human-centric view of consciousness. Thomas Nagel has a famous paper called, "What's it like to be a bat." Even as a human if I know every scientific factoid about bats and I imagine myself to be a bat, would my humanity allow me to really mimic the mind of a bat? No. I can only do so through a human lense. I am imprisoned by it. I have no idea what it's like to be a bat despite knowing what echolocation is. I can't really tell if bats know they are bats. I can't tell what it's like to experience the world as a bat.
In other words, it might not be that only a few species are conscious. It may actually be that we have a very narrow understandig and myopic ideal of "consciousness." Dogs aren't conscious, supposedly. Dogs have no interest in their reflection for the most part and that kind of thing; but, every dog I've ever had seemed to realize that I was different from them. That I was not whatever they are, and that I had the ability to do things they could not. They would even play to my sensibilities. They immediately recognized others from their own species and vice versa. A dog isn't going to mistake an alligator for another dog, for example. In my observations of them, dogs seem to know that they are "dogs." They may not know they're referred to as dogs like I know I'm referenced as human, but they don't seem to conflate humanity and the canine existence.
Simply put: we don't know enough about consciousness to determine what animals are and aren't conscious.