Over the course of a long, punchy New Yorker essay, Sasha Frere-Jones asks why popular music—the music he follows, at least—has become so self-segregated. There's some worthy, overdue bashing of Yankee Hotel Foxtrot (synthesizer squeaks and echoey feedback… fail to give shape to the formless music), and there's this theory about how anti-sampling lawsuits clipped the rock-rap umbilical cord.
Beginning in the late eighties, there were several high-profile lawsuits involving sampling. In 1991, a U.S. federal court ruled that the rapper Biz Markie's use on his album "I Need a Haircut" of a sample from a song by Gilbert O'Sullivan constituted willful infringement. (The album was withdrawn from stores and rereleased without the offending track.) A similar suit led to a decision by a federal appeals court, in 2004, that the use of even three notes from someone else's work could be a violation of copyright, making it difficult for all but the wealthiest rappers to use samples. For twenty years, beginning in the mid-eighties, with the advent of drum machines that could store brief digital excerpts of records, sampling had encouraged integration. (Think of De La Soul rhyming over an excerpt from the seventies educational cartoon series "Schoolhouse Rock!" or of Jay-Z rapping over a snippet from the Broadway musical "Annie.") In practice, the ruling obliged hip-hop producers to write their own music, which left them with a larger share of royalties. And, as producers became as powerful and as well known as rappers, having a distinctive sound that wasn't associated with another genre or artist became an asset. Rap musicians, lacking incentives to appropriate other sounds, began to stress regional differences instead: in Atlanta, the rugged, spare sound of crunk; in the Bay Area, the whizzing, burping, synthesizer-dominated sound of the hyphy movement.
This seems true: I can think of popular exceptions, but they're usually suburban rappers like Kanye West and P. Diddy throwing a bone to the white boy rock they liked growing up. Sampling is by no means dead, of course, and Frere-Jones doesn't stop theorizing there.
Jesse Walker on mash-up sampling here.