The issue with the last duos was that they did not have names. Giving the products fancy, silly, or sexy names is part of the MAC branding. It doesn't come as a surprise then when a product named "Duo 1" will not sell well. Who wants a generic name if you can have a fancy one as Marche aux Puces?! Big marketing fail!
Another issue was (and still is) that fashion trends are moving away from eye make ups with too many colors. In the past year or two, we have seen increasingly a very different application of e/s, i.e. only one or two colors, often taken up to the brow bone.* It seems that it takes people much longer to adopt this trend. And with this new fashion, the duos came along. I'm not saying that multi-color e/s make up can't be beautiful, but it has become an old hat. The new trends are much more subtle, but still very intricate and detailed in their application. Contouring the eyes with neutrals is really an art. I love it!
* Alternatively, we have also seen quads were the colors are very similar rather than contrasting.