I think it's more marketing than anything else.
A brown leather grip is consistent with the image of a "true" traditional tennis racquet. To the average tennis player and new comer to the sport, it conveys an image of seriousness, tradition, and most of all quality.
From a business/marketing perspective, there should only be one "top of the line" product/range. Having too many top dogs can confuse the customers who aren't prepared to do the necessary research. And a confused customers is one who won't buy either products.