I don't know but I would suspect that sales have dropped on covers for the DX since the smaller and more recent Kindle has surpassed it in popularity. There's just not much demand for the DX and its accessories. However, I do love my DX and its red cover. The cover is of excellent quality. It is sturdy and has held up well.
What colors are you looking for? There is a multitude of colors out there for the Kindle DX. On Amazon they have Blue, Brown, Red and Green available. There is also vinyl skin covers available in many colors. I have the red cover and I love it, it is sturdy and still looks great.