Because it set the blueprint for all modern rap albums that have come after it. If you listen to the lyrics on illmatic nas touched on almost all subjects that are prevelant within the hiphop community today. From drugs to bytches partying, fashion, stuntin,weed, hood stories, introspective shyt, personal shyt. Almost anything anybody raps about today nas already talked about it on illmatic. And he did it with a lyrical style that has still not been matched to this day.