Could it be because white and black are free colors? Every other color is $550. Plus, if a dealer is ordering it for the lot, they take less risk when they order white or black. Other colors are harder to sell as CPOs.
We just took a tour at the x3 factory in SC and they stated that White and Black are their most popular colors. I saw a lot of those on the line, next most frequent were the grey shades. Saw one brown, maybe three blues, and one or two reds.