Biggie was known in some of NY before the album dropped. Party And Bullshit was a local hit in Brooklyn and the Flava In Ya Ear remix introduced him to many people. Most of the harder tracks were recorded first and it took Diddy awhile to convince him to do the singles. At the time Bad Boy was running shit in NY and Diddy knew all they needed was a hot single.
Juicy was a huge hit, rap wise and mainstream wise. The single included Unbelievable for the streets. Then Big Poppa came out which was huge too; once again the single included another street track, Warning. So he had all bases covered before the album dropped, he had the charisma, the right label, and the right sound.
Whereas Illmatic bombed earlier that year. It made an impact in terms of critical acclaim, receiving five mics in The Source (iirc Ready To Die got 4 mics). It was also perhaps the first rap album with a super team or producers. Every hot NY producer was on it, and that influenced Ready To Die and other albums. But the album didn't have any mainstream singles and was heavily bootlegged; I've heard stories of nearly a million bootleg copies being circulated. Plus if you were a rap stan you had heard half the album before it came out.
Final thing: Nas wasn't nearly as charismatic as Biggie and the song content wasn't mainstream. He was and still is quite reserved. Questlove has told a story about seeing Nas lose every award at the 1995 Source Awards to Biggie, and how at that moment he knew Nas would do something drastically different next time. Then 1996 came, Nas dropped a heavy mafioso/mainstream album and sold three million records+had a hit single.
