T'Challa is just a character. There have been countless Batmen, Supermen, etc. They switched out Ed Norton for Mark Ruffalo with the Hulk and Don Cheadle for Terrance Howard at the beginning of the MCU.
That's not the same thing at all man. In the MCU neither Hulk nor War Machine are remotely on the same level as BP. It would be like saying "why didn't they just recast Ironman?"
Which they flirted with the idea of....until fans went in on them. Then they buried that shyt and continued to pay RDJ a stupid amount of money.
Fred.