It depends who you ask. Suge Knight claimed in a 2012 interview it was because Dr. Dre was a closet homosexual. Daz Dillinger, a fellow artist, said it started when Dr. Dre lied to Tupac about producing “Got My Mind Made Up.” Apparently, Dr. read more
Also, Dr. Dre has allegedly had ghostwriters from the time he was a member of N.W.A. to his last album. This includes when he was at Death Row, where he released “The Chronic.” At the end of the day, Tupac dissed Dr. Dre multiple times when Dre left Death Row and Dr. Dre never responded. read more
Anyway, I think the reason why the Tupac and Jay-Z beef wasn't looked at so heavily is because Jay-Z was a nobody at the time. Also, most people probably looked at it as Tupac getting at Biggie's camp (a.k.a. getting at Biggie). read more