The answer is simple. LL was by far the most commercially successful of them in the 80s. He was very comfortable stepping into the contemporary pop lane. He became a major celebrity outside music due to his acting career. And he got by far the most label support. Dude was getting top beats from top producers and had expensive videos in constant rotation.
G Rap never had much commercial appeal. By 1998 he sounded the most contemporary of em all because so many of the top NY rappers was obviously influenced by him, and he was killing guest spots with younger acts left and right but never had a decent label situation. At least he got a nice check from Rawkus.
Kane had some memorable guest spots too but that 1998 album he dropped was
obviously done on the cheap and got no push.
Rakim never really updated his style. The 18th Letter was cool and did well, but The Master was a kind of like the Kane joint,
and obviously done cheap with minimal label support. Aside from the great Premier single. Supposedly Dre wanted him spitting gangsta shyt on Aftermath
Slick Rick dropped a successful and contemporary sounding album in 1999 with a bunch of next generation features he sounded at home with, still don't understand why he never had a follow up. This is the newest album from any of em that's actually good and worth going back to with any consistency