Because stuttering is a speech impediment. Neurodiversity refers to disorders that some people think are non-pathological, like autism and ADHD. Meaning that these people think autism isn't a disorder, just a different way that people behave. Meaning that they don't believe it should be treated and should just be accepted. In short, the movement rejects the idea that these disabilities should be medically treated. Personally I don't agree. The movement is harmless for people that have mild autism, but if you see a person with severe autism you can see that it isn't helping them at all because they really do need help. The same, in my opinion, applies to severe stuttering, because it clearly impedes basic functions of human life.