It might have been a studio move, because you probably don't want your white child protagonist referring to the black man acting as his servant (that changed further on in the series, becoming more of a bodyguard) consistently as 'butler' only.
Of course, why Butler was black in the first place instead of 'Eurasian' like he and his family are described as being in the books is a wholly different question, seeing as his place of being raised has a certain importance to his upbringing. I don't claim to fully understand how studio executives go about casting characters, though.