Its because black people are viewed so much cooler than everyone else. Naturally we all want to be viewed as "cool". Society sees black people are cool so we try and act black. When was the last time you saw anyone pretending to be asian or white? it doesnt happen because society views asians as hard working/nerdy or terrorists.
PS, these are not my opinions, im just stating how i feel that society dictates what is cool and what isnt.
My sig shows a black guy doing something extremely cool. If clapton did that, everyone would be like, "wtf is that white guy doing?"