Apple first got rid of it by going to the Unibody design. Then they decided they wanted that design in the MacBook Pro and a 13" MBP was released. They wanted the MacBook to look like a cheaper, more economical computer, so they made the distinction mainly in casing. Now you have it as it is.
Before, Black Macbooks were traditionally priced a couple hundred above the white Macbooks. Now, a couple hundred above the white Macbooks is the MBP 13" so I don't see where they could fit a black Macbook in. Plus, white is Apple's flagship colors (not sure if it is anymore)