The answer to this question can be found in Chapter 16, which details the author's first foray in to the world of working as a black "help" in a white house. Mrs. Cullinan, her employer, is quite happy calling her Margaret to begin with. What changes this however is when she has some of her friends over and one of them suggests that she starts calling Margaret a different name that might be more manageable. Even though Mrs. Cullinan describes her, rather patronisingly, as a "sweet little thing," note what her friend advises her to do:
Well, that may be, but the name's too long. I'd never bother myself. I'd call her Mary if I was you.
Mrs. Cullinan therefore changes Margaret's name because of the recommendation of her friend, who thinks that the name Margaret is "too long." Clearly, she cannot be bothered to expend so much energy uttering the three syllables of Margaret when she could merely say two syllables instead. It is of course yet another example of white supremacy and racism that the protagonist in this autobiographical novel encounters in her life.