Final answer:
Mayella Ewell lied about Tom Robinson because her claim of being assaulted by a black man was a socially acceptable excuse to explain her actions, and to protect herself from the community's backlash for breaking a powerful taboo.
Step-by-step explanation:
Atticus Finch, a key character in Harper Lee's novel To Kill a Mockingbird, believes that Mayella Ewell lied about Tom Robinson because admitting her feelings for a black man would have caused her to be ostracized by her community and her own family.
Atticus suggests that Mayella's accusation stemmed from her desire to purge herself of the guilt and shame she felt after making a romantic advance toward Tom Robinson. Her actions were driven by the societal norms of the time, where interracial relationships were taboo and could ruin a white woman's reputation.
Essentially, Mayella Ewell lied to avoid the social repercussions of her unconventional feelings, despite the cost to Tom Robinson's life.