Disability is seen as a fate worse than death. Persons with disabilities have been hidden away in attics or institutions of confinement, or, in the worst case, killed. This fear, mostly never articulated, leads people to feel uncomfortable around people with disabilities. Or even to somehow find ways to deny a disability exists. In our society, people with disabilities are mostly on the margins.
Disability is, after all, a common condition of life.
Most people, sooner or later, become disabled in one manner or another. People with disabilities come from all races, all classes. If you imagine 'the disabled' at one end of a spectrum and people who are extremely physically and mentally capable at the other, the distinction appears to be clear. However, there is a tremendous amount of middle ground in this construct, and it's in the middle that the scheme falls apart. Functionally, there may be little difference.