Think of it this way: What if they made a movie about George Washington. Washington was a white general. So would it make sense to portray him in a movie with a Latino, African or Hindu Indian?
Same with Wiz of Oz. It was written in the 20s by white people portraying white fictional characters to other white people. In the 1920s, audiences were probably 98% white.
So when a movie comes along using an actress who isn't just a little different but is DIAMETRICALLY OPPOSITE to the original story, one has no choice but to believe this was done deliberately to make a political or racial statement, which is not at all in keeping with the original story.
I actually read the actual book Wizard of Oz by Frank Baum before I ever saw the film.
Seriously, the range and opportunities out there for stories and roles is almost unlimited--- if someone wants to cast a film with blacks, why do they have to pick on stuff already done and written for a white girl and not simply write something fort blacks?
I suspect the real answer is that because they know it would not sell and would be a huge loss at the box office.