Nas vs Jay z lore goes way back and much deeper than refusing a feature, first you have to understand the context of the era that nas/jay was coming up in and how detrimental it was if your character could be called into question unlike today(Drake, Rick Ross, ect). Let me explain, Jay z image was already tainted early on by him acting like a clown in jaz-o videos and even worse a few years later, he pops back up and all of sudden he is rapping about being a big willie cat in brooklyn which rumors over the years have stated that Nas views that whole story as fabricated non-sense by a rapper desperate to get over after making a joke of himself with jaz-o failed, and that no legit brooklyn ***** could ever vouch for jay z hustling tales, I'm sure Nas asked AZ if jay was legit in BK too.
Nas's mind was most likely already made up probably before he even met Jay, and simply, he didn't want to associate with a fraud. Jay on the other hand like majority of the NY rappers circa early-mid 90s loved Nas's whole style and was influenced by it. Jay as a Nas stan tried over the years to get in Nas's good graces even going as far as sleeping with nas baby moms just so he can feel how it is to put his dick in same pussy that nas dick touched, but to no avail. Frustrated that Nas refused his friendship, and showed the camal no respect, Jay took it upon himself to start the beef. Tbh, jay just wanted some acknowledgment from his hero and nas simply didn't care too,