"By the 20th century, the breed's distinctive markings and color were written into the standard, becoming an essential feature."

Source: The Dog Book by James Watson, 1912

Counting for over half of the points awarded by a judge in evaluating a Boston Terrier, the Boston's head and expression are the most important breed characteristics, giving them the true "look" of the American Gentleman.  This expression cannot be achieved without exactly what the standard calls for  - Eyes that are large, dark and round, nose that is black and wide.  Contrast this to the expession of the colored Boston below below and you will understand why the founders of the breed likely narrowed down certain colors to carry on the vision of the breed while in its infancy. 

Contrasting Expression of the Colored Boston Terrier

correct eye/nose color

"The eyes are wide apart, large and round and 

dark in color." 

"The nose is black and wide"