Thank you for the opportunity to help you with your question!
Society of the time and certainly the part of the country made interracial romance strictly taboo. With a strict and overbearing father, Mayella saw absolutely no alternative but to falsely indict Tom Robinson. Mayella knew that she had made advances and even kissed Tom and because of societal norms felt both guilty and shameful. This led her to see no alternative but to use Tom as a scapegoat.
Please let me know if you need any clarification. I'm always happy to answer your questions.