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.
I think all of that was him being directed to playing out a role as a Master Jedi. You have to remember before those movies the only actors that were able to really flesh out how a Jedi acts is Alec Guiness as obi wan and a puppet name Yoda. It was new territory. So you get this plain mannerism that is calm but absolute. Samuel was trying to convey that somehow and it just came out odd, probably partly because it was motherfukkin Samuel L Jackson doing it. Where as when Alec Guiness did it, the oddness just worked with the actor.
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.