There’s not much reason for that though, other than that was kinda the default at the time so not much thought was put into that. Besides the main cast, you could very easily have swapped most every other player in the OT with someone of a different gender/race. Honestly, I’d even make the argument that you could swap with much of the main cast too (Lucas was at one point considering the idea of Luke as a girl, after all).
The idea that Lucas purposefully casted almost all white dudes is kinda silly, really is was ultimately something of an unfortunate oversight.