Because the act of writing aboout and recollecting her experiences on Mango Street makes her realize that she is strong enough to leave the area. Esperanza's narrative ends in a circular fashion; however, Esperanza's progress from the first vignette, whose first paragraph begins with the same words of the third paragraph of the last vignette. Compared with the first, the last sketch presents a much stronger "I" who clearly distinguishes herself from the communal "we" that characterizes the first story. In addition, in the final skecth, the narrator says that what she remembers most is not moving houses as she says at the beginning of her narrative, but Mango Street, implying that it is now only a recollection and that she has moved elsewhere in a house which is her own ("Not a man's house. Not a daddy's" as she puts in "A House of My Own"). Yet, to move away from Mango Street, she has had to accept it as part of her own life. This contrasting feeling is conveyed by the sentence "the house I belong to but do not belong to" on the last page of the collection.
Thus, in your theme statements, you could analyze the ways in which Esperanza belongs to Mango Street and the ways in which she doesn't belong to it. The narrator celebrates her links to the folk elements of the Mexican American tradition and challenges white Americans' stereotypes about Mexican Americans. Yet, her gender and her education make her critical of the role women are confined to in that society ("My Name" is an interesting vignette for this theme as are "Boys and Girls" and "There was a Woman She Had so Many Children She Didn't Know What to Do"). You could also analyze how the collection contrasts Esperanza's own development with the immobility and stagnation of her community. The genre of the "coming-of-age" narrative thus supports the main thematic concern of the collection. Finally, you could also focus on how Esperanza distances herself not only from Mexican American males, but also from those females who cannot rebel against the impositions of a male-dominated society.