I did some research years ago into the influence of rap and gang culture in popular media on teens: when they're in the early stages of their career such artists have to adhere to a relatively narrow image and behaviour set, a generic hard-man, brotherhood & ghetto image where authority of any kind (education, law, social norms) doesn't apply to them. It's effectively a studied ignorance, which in many cases does belie intelligence, imagination and a wide scope of view - many, such as Ice T, 50 Cent, Snoop Dogg (Lion?) etc once they became established allowed their actual talents and personalities to come through, branching into different kinds of music, writing, acting etc, at which point they're described by many of their genre fans as "sell-outs".
The basic rapper image is one of self- and group-empowerment among people from an otherwise disenfranchised demographic. It gives them a sense of self-worth and belonging, which is what many young males especially crave: and the generic image of hostility to authority and disrespect towards/disregarding societal norms outside of their own group identity is a powerful one, to which all who identify with the culture must adhere quite closely. It doesn't preclude articulacy at all, but what they actually say has to be within surprisingly narrow margins. It's not just rappers of course: the same applied to punk. Watch early Sex Pistols and try to reconcile that with what Lydon was turning out 10 years later. Only the voice remains the same.