IMO, it's because the black of the original logo takes up space, and keeps the BMW text and the inner circle grounded to a larger context.

Without the black, the text and the circle are just hanging out at the top of the outer circle, unbalanced against a large empty space at the bottom of the outer circle.

EDIT: Another way of putting it is that it looks like the text and inner circle could just bounce around inside the outer circle—they are "loose", and, as I said above, ungrounded.