How can you say this has never occured to any other type of music? What about post Jazz rock-n-roll? Rock music in the late sixties and early seventies developed as protest music. Almost the entire genre was born out of protest against power and the Vietnam war (yes, I know rock was invented by black musicians but I am speaking of a particular genre that we are most familiar with). "War, what is it good for?", Hendrix's rendition of the Star Spangled Banner, nearly everything by John Lennon, early music by the Rolling Stones, Pink Floyd's "Another Brick in the Wall", and the list goes on. The only problem was that as this music became so much more popular, the only way for artists to get produced and distributed was through the major record labels that owned access to the industry. Of course, at first, the labels did not want to miss the profitability boat that was this music, but as time went on they were able to mold artists to fit an image that was friendly to corporate america. The early protest artists that signed on to these labels soon learned that their music only enriched those who they were protesting against. As Warner, Columbia and other labels continued to collect money off record sales that money was used to pay sponsors who were building weapons for the war. Soon after came the whole culture of "sex, drugs, and rock n' roll" which taught the values of materialism and individualistic gain. Just as the bling-bling of today is completely in line with the values of capitalism, so did the culture of glam rock. You can see the same progression in punk music that has digressed from the Dead Kennedys in the 80s to Sum 41 today. This trend is not at all particular to hip hop. One good article to check out is "Rockin' Hegemony" by John Story. It lays it all out perfectly.