When a person loses one sense, they often gain sensitivity with another sense.
For instance, a deaf person often becomes more alert of their surroundings.
For a person who is blind, they rely on their sense of touch and hearing.
For a person who is deaf and blind, they rely more on their sense of touch.
I knew a woman who was deaf and blind, but her sense of touch were amazing. I used to interpret for her at church until she moved elsewhere and I moved elsewhere.
One day, I was at Six Flags for Deaf Awareness Day. I was with a friend and I saw this woman walking by (with a guide). I told my friend that she would recognize me after 5 years of not touching me. My friend thought I was joking. So, I approached her and her guide asked me for my name so that he could tell her who I was. I told him "no" and gave her my own hands. Within seconds of touching my hands, she immediately knew who I was. Both the guide and my friend were shocked. (No, I don't have warts. No, my hands are not dry. No, I don't have ugly wrinkles.) She just could tell who I was by feeling my hands.
Anyways, my point is that... for a person who isn't blind, learning to understand ASL with hands only is difficult. Even when we try to learn, it's still difficult because our primary sense is to see things around us. For a person who is blind, they rely on touch. So, it becomes easier to understand sign language with hands.