Because only roughly 5% understand what it is all about. The rest, roughly 95%, use opportunities like that to show what great justice warriors they are, but failing totally to represent the wishes of the disabled correctly.
I am sure you feel differently regarding your son. But as a person with a disability, I can tell you that this never worked in my family. My mother might be motivated to do all sorts of things, but most of them are not helpful or even subtly counterproductive.
To sum it up: If you speak for someone, you are likely patronizing them.
I'm conscious of that. But my son literally can't speak for himself so if I (or my partner) don't speak in his interest, who will? And while other people with disabilities can speak for themselves better than I could ever hope to, there is a difference between amplifying / supporting their concerns and being patronizing.
The sad reality is that the mainstream does not want to think about the challenges facing disabled people and they are all too happy to simply ignore the (perceived) minority which is directly impacted by them. All these absolute assholes who park on parking spaces for the disabled come to mind. Those people will only ever be convinced if they are not confronted by a broad section of society. It is simply not something that a minority can ever hope to achieve without the explicit support of at least some part of the majority.
I am sure you feel differently regarding your son. But as a person with a disability, I can tell you that this never worked in my family. My mother might be motivated to do all sorts of things, but most of them are not helpful or even subtly counterproductive.
To sum it up: If you speak for someone, you are likely patronizing them.