From the Wiki: According to former Death Row Records chief engineer Rick Clifford, George Lucas was eyeing Tupac to star in his return to the “Star Wars” saga. Lucas had asked Jackson to ask Shakur to audition but due to Shakur’s untimely death the role ultimately went to Samuel L. Jackson. The character’s bald appearance could have also been an homage to Shakur.
It also just doesn’t play to Jackson’s strengths at all, which are mostly in his skill with dialogue and emotion. I don’t blame him exclusively for it, but it was unquestionably a terrible acting performance that was delivered.
It was probably dozens of people’s faults, of which he is just one.