No such thing as stupid questions

I have heard it called "John Lennon's murder" but can't remember hearing it referred to as an "assassination." But assuming the premise of your question is valid, i can think of two potential influences:

(1) "assassination" is most often used when talking about politicians or state actors and Lennon was more of a political figure than 2pac; (2) 2pac knew he was likely going to die the death of a "gangsta"--i could reference almost every one of his tracks--this mentality was associated with a gang affiliations, and had a lot of racial implications. It would be hard to imagine a journalist who identified with this mentality, and harder to imagine one that referred to the death of a gangsta as "assassinated," who are generally "killed" or "murdered."

Tldr: assassination is too fancy a term for the death of a gangsta.

Ps: favorite 2pac track "death around the corner"