Earlier quoted context omitted.
Because there is a difference between "deaf" and "Deaf". The capitalized word is meant to highlight the cultural and social parts of living in a community with other Deaf people. You also would not write American in the lower case. Being "deaf" is about the handicap, about the loss of hearing and other problems. Being "Deaf" is about being a member of a community, about the keen visual sense, about communicating in a…
So there are Geeks, Nerds, Programmers now too? Why not capitalize every Noun like in German
+it’s almost 100 degrees here