Because (again, not to be racist) it is geographically and biologically impossible for samurai to have been black, since all Samurai were Asians, specifically Japanese (of course). Technically, making a Samurai a black person is racist to both Asians and black people. There is a loophole, however, since I'm pretty sure most of the Assassin's Creed games take place in simulations. So, the whole black samurai thing would be the least of our concerns, and isn't even consequential if the game you're talking about also takes place in a simulation, since the fact that the world is a simulation nullifies any historical indescrepancies that may be present.