Lyrically, both broad genres share more similarities than their listeners would like to admit.
We've often heard the following response to the question concerning a person's favorite type of music. "I like pretty much everything except country and/or rap." Why is that? Well, the artistic expressions of culturally alienated groups have a lot in common. Many of country and rap listeners have been either geographically or socially excluded from access to education and corresponding economic opportunities and respect. When a group is alienated like that, its members tend to split their musical expression into two broad categories:
1) Pride/autonomy/self esteem building musical expression at the expense of the non-alienated group. The will to power of culturally marginalized people is manifest with efforts to turn perception of the disadvantaged position into one of a prideful advantage. Whites in poor rural areas and blacks in urban ghettos use music to see themselves as more in tune with life and reality because of daily hardship. The music stresses that the disadvantage of rougher less materialistic life is an experience that produces stronger more independent individuals that know how to live. Knowing how to live of course is shown through constant supply of physically draining leisure activities, intoxicants, and struggle for a more promiscuous status. That of course is not a drastically different emphasis on proper life compared to pretty much every other group in the social sphere.
A song recently played on "new country" radio station perhaps best illustrates the self esteem building found in resentment based music. It starts out with a singer going "this goes out to all you sophisticated ladies out there". It proceeds to tell a story of a girl, from presumably sophisticated Los Angeles, who moves to the mid Western college to study law. Right away she forgets all about the boring urban aspirations and ends up having the time of her life riding in a pick up truck with a wild eyed cowboy. The message is clear. Alienated people not only sexually gratify better ( than the stuffy, boring, effeminate, and overworked urban middle class men ), but actually possess knowledge of what real fulfilling life is about. In addition to this wisdom, outlaw country and gangster rap boasts of living by one's own rules and being autonomous of broader social influence. That is fine and dandy in itself and shows a healthy logical way for a group to improve the feeling of its influence in the world. However, music like country and rap/R&B is also thoroughly focused on contradicting its own ego propping message.
2) Musical expression of lamentations, sorrow, and depressing fatalism. R&B and country are very good at revealing, in often intricate detail, how the daily life of the alienated group is just plain unpleasant. There are endless lyrical stories of fights over money, broken relationships, struggles of single mothers to support the kids, wives leaving husbands for somebody better off, lost jobs, and substance abuse. The lyrics of poor urban blacks and poor rural whites overwhelm with the most depressing realism based ballads. Country and Rap/R&B are just the real deal goth without the make up and exaggeration. This broad stroke excludes the current materialistic party rap that is just the rap equivalent of 80s big hair rock bands. The party rap period will enter its grunge equivalent stage as the international economic depression continues.
Considering how a large part ( if not the majority) of lyrics in country and rap/R&B are consistently describing how horrible the lives of their listeners are, the silly lyrical prideful chest beating just brings attention to the depressing underbelly. Lyrically, both broad genres share more similarities than their listeners would like to admit. The contradictions, resentment based identity building, and realistic unfortunate scenarios of strangers is exactly why people make a point to exclude these two types of music from their answer.