Perhaps because the character is "not" really necessary in the book... and was a mark of JA's relative inexperience when she wrote it (in that Marg has very little "to do"). Some critics have theorised that Margaret was more important when the novel was epistolary....also the plot required Mariann and ELinor to go to London together, so a younger sister, to keep Mrs D company was if not exactly a necessity, useful for the plot? But the first adapation of S&S I saw in the 1970s cut Margaret altogether and I dont think that it made any difference........