It's all about consistency. Nas's singles didn't rule the Top 40, but he has three albums that sold 2 million copies, three that sold 1 million copies, and three others that sold 500K, and one that is just shy of 500K. I can't think of many rappers who have been able to do that especially within 20 years.
Jay Z has consistently had hits throughout the course of his career and his biggest have come at the latter end of his career. Probably won't see this again either.
That being said, both are successful because they had consistency vs. being a one hit wonder or having a 5 year run and no longer being relevant beyond that point.