Snake,
I might be wrong, but I feel that you are ignoring, making light, or are not aware of the history behind the n-word. It was one tool in an arsenal that was used to stigmatize and disenfranchise blacks in this country.
Using the n-word is a 'big deal' because of this history. Whereas whites have generally been in a position to not be defined by the label cracker or redneck for their entire lives, blacks have not. No matter what your level of education, pedigree, etc., back in the day all blacks could be considered or called the n-word, and were subject to the lower status that word symbolized. I don't think it was the same for whites, in the past or now.
Black comedians 'make' race a central part of their routines a lot of times, because race is a central part of their lives, and more than likely the lives of their usually predominately black audience members. Whereas for white comedians and white audiences it might not be something they have to think about much at all, blacks often aren't as fortunate.
Black comedians do skewer whites, Asians, Latinos, Arabs, and other blacks as well. Sometimes many take it too far, but I would posit that there are white comedians, Latinos, Asian, etc. that also make racial jokes. Especially since it is cool to not be "PC" anymore (ex. Howard Stern or that Mind of Mencia show). And there is definitely a history of anti-black racial humor in this country, i.e. minstrel shows and blackface. I also want to note that the 'target' of most black comedians social barbs are black people themselves.
I do agree that the labels against whites don't carry the same weight. But that is because whites are the most powerful racial/ethnic group in this country. A black person calling a white person a cracker is about the same as a gnat buzzing by the ear of an elephant.
With the n-word there is a whole subsequent history there to contend with of power relationships between the two races that isn't there with cracker. Now, among white people themselves there might be more of a class/cultural/regional issue there in the usage of such terms as redneck, cracker, or white trash.