I do not know why specifically but it is probably because there are so many different variations of splash. One bird could be splash and be completely white with only one splash of blue. Another bird could be almost entirely blue and showing only a bit of white or darker color. It's up to the judges discretion and preference. With the other colors, such as blue, black and white, there are clear definitions of what is desired. Some breeds, splash is accepted, I believe Silkies are one of those breeds. You can show a splash bird in AOV (All other varieties) but will not place higher than the best of variety.
With splash, there's simply no way to regulate unless you go by percentages of body colored a specific color, areas of the body colored, etc.