I think it’s because we physically have difficulty speaking when it’s something that comes second nature to 99% of people, even if there are other disabilities present. Most people don’t understand it and don’t have experience with out outside of a kid they knew in high school or a character on TV, and is usually comic relief or a side character that never get truly explained. It’s usually seen as something we can “work through” and virtually “cure” and it’s our fault if we stutter. Not like other disabilities where there is more empathy nowadays. Everyone wants to help the kid with downs, or the kid in a wheelchair, or whatever else, but the kid who stutters needs to “work on it” but it’s really just misunderstood.