Better yet, why not have a national bar exam? Some people will say "Oh, but if you don't pass a particular state's exam then you haven't demonstrated knowledge of that state's laws." Well, lots of states currently have reciprocity agreements that allow a person admitted in one state to be admitted in the other without such a demonstration of knowledge. It doesn't seem to cause any problems.
Can you imagine the plummeting bar pass rates, however, if we adopted a national bar exam based on the California bar's format?