Continuity mistake: At the end, Scar walks opposite Simba on top of pride rock. He approaches the embers on the floor. We then see him approach the embers again from a greater distance.
Continuity mistake: When Mufasa has to teach Simba a lesson it's still very light all around. By the time Simba walks up to him it's almost dark.
Answer: None of the cast of The Lion Guard was cast in the film, so why would Oyelowo have been? Ejiofor was cast because, in the words of director Jon Favreau, he "is just a fantastic actor, who brings us a bit of the mid-Atlantic cadence and a new take on the character. He brings that feeling of a Shakespearean villain to bear because of his background as an actor. It's wonderful when you have somebody as experienced and seasoned as Chiwetel; he just breathes such wonderful life into this character."