Indeed - there were generalizations being made about rock, as there were about rap, that were necessary to make a point. I'm basically just talking about what's "popular" right now, mainstream stuff. As this thread has shown, gangsta rap is by no means representative of rap as a genre - it's simply what's selling right now.
In terms of connecting to and influencing an audience, I think rap music has something over other genres. Throughout much of its history, it's told the stories of the downtrodden, the stories of inner-city Black people and what they have to go through and experience, something that wasn't represented in the music world. To have music that finally relates to your situation, something you can really connect to on a basic level, is pretty rare, and so is quite powerful.
What I'm trying to get across is kind of difficult to explain. For most Black people growing up, pretty much every medium - television, movies, music, even print - was dominated by the majority culture. Basically, you have to imagine looking around and seeing hardly who looks like you, who's had similar experiences as you - trust me, it's an alienating experience. That alienation, combined with frustrations about inner city life, was part of what brought rap music around in the first place. To finally see your people making songs about your situation and the problems you face -- problems that America wants to ignore and sweep under the rug -- and seeing it in a musical style that you can call your own - that's heady stuff. Giving voice to the voiceless - that's what made rap so powerful.
That power and influence is still present in the inner cities of America. Most inner-city Black kids are pretty much listening to hip-hop as their main genre. The problem is that now the messages of the mainstream acts are all about sex and drugs. I'm not saying that rock hasn't been the same way (again, Motley Crue, GNR), but the way it's told is way different.
The Crue never explicitly said "I sold drugs on my way to the top, it's how I got quick money" - many gangsta rappers, on the other hand (Biggie, Jay-Z, 50 Cent, WC, just to name a few) have essentially said that exact thing. And the kids hearing this are seeing the drug dealers every day, driving around in Benz's and BMW's, so it's just getting into their heads even more that dealing drugs is a great way to get easy money. Never mind that it's also a great way to get DEAD.... Without getting into all of it, the main problem is the glorification of those problems (drugs, violence) which are plaguing the inner cities. And don't even get me started on the misogyny... "boys will be boys" is one thing, but rock music was never as overtly explicit and derogatory as mainstream rap has become.
Of course, there are lots more factors involved on many different sides of the issue, but i'm trying to keep it relatively short (LOL). So yeah, that's why rap so damned influential.