There seems, to me, to be two great advantages to Hip Hop: that one doesn't need to have a good voice, and that much more content can legitimately be crammed into the lyrics. The first point is fairly self-explanatory, although I'll admit that it's not an immutable law: neither Anthony Keidis or Peter Garrett had a good singing voice, but both have been successful singers. Still, regarding the second point, consider two songs: Nice to Know You, by Incubus (a Rock song); and Full Moon, by The Herd (a Hip Hop song). The former song has 137 words; the latter 509, 3.7 times more words than the former. You could also consider the words per minute, which is 29.1 for the Rock song and 110.3 for the Hip Hop song, making the Hip Hop's 3.8 times larger.
Is this a good thing? I don't know, but I think anyone who's ever written an essay with a 1000-word limit would agree when I say that it's easier to say what you mean when you have more time to speak. One could argue that it takes much more skill to deliver a message with fewer words, and I'd agree; but I'm of the opinion that the message itself is more important than the delivery, so if it can be delivered better with more words, then it should be. Bob Dylan or Paul Kelly may have been able to depict the moribund outback town more forcefully, but it's unrealistic to expect every lyricist to be a Dylan or Kelly, isn't it? My real point is that Hip Hop has the potential to give much more lyrical bang for your proverbial buck.
It's also strange to consider the differences in content that you see between Hip Hop lyrics and other genres'. It's OK, for instance, for a Hip-Hop artist to write a song solely about how good a lyricist they are. You can hear these songs everywhere, it seems like Scribe (a New Zealander Hip Hop artist) writes songs about nothing else, with ostentatious titles such as "F.R.E.S.H.". This is done in other genres, sure; but when it is, it is much less acceptable to an audience. So even if Hip Hop does give a greater opportunity for a message to be delivered, it is often shamelessly squandered.
Also, it seems like a much greater degree of transparency or candidness is expected of Hip-Hop lyrics; or perhaps I should say that they're the exception to the general rule that lyrics don't necessarily need to make any sense. Most non-Hip-Hop lyrics that actually mean something are brimming with obscure imagery and seemingly unnecessary phrases, so that many of them can be said to make sense in the same way that Finnegans Wake can. OK, perhaps that's an exaggeration, but Hip Hop lyrics are definitely expected to be much more immediately comprehensible than others, so decoding or analysis is usually unnecessary. Perhaps this isn't even a good thing, I'm not in any position to say.
It seems that the occupations of composer and poet have intersected to some degree: mainstream instrumental music is extremely rare nowadays, whereas 150 years ago it was the reverse. So in light of the amalgamation of poetry and popular music, I think of Hip-Hop lyrics as being a divergence from that: it is more like rhyming prose than poetry. You wouldn't get away with saying "Perspective pries her once weighty eyes and it gives you wings" in a short story, nor would you in a Hip-Hop song, but it's the norm in other genres.