Largely because this is what the English desire of it. English mustard is made with equal quantities of white and brown mustard seeds; while the white give heat on the tongue, the brown give a volatile heat in the back of the throat. It is usually sold dry and then simply mixed with cold water, which adds to its strength of flavor.
French Dijon mustard is made from brown mustard seeds only, diluted with white wine and spices to make it less hot than English mustard. The sweeter, smoother German and American mustards are made with white mustard seeds, their flavor softened by additions such as sugar or herbs.
** Free Recipes