The Ewells know that they are the lowest of the low amongst the whites in Maycomb. They have no money, no education, and no breeding. The single thing that elevates them at any level in the community is the fact that they're white. Like most people in similar situations, Bob and Mayella would like to better their station in life. However, Bob is unwilling to put forth the effort necessary to change his family's lot and Mayella doesn't have the resources to change her own life.
With her mother dead, Mayella becomes a surrogate wife for her father and mother for her younger siblings. The fact that Mayella wants a better life for herself is evidenced by the red geraniums she grows so lovingly — they're the only sign of beauty in a dismal, filthy shack and yard. She can't attend school because she has to take care of her younger siblings, especially when her father leaves on days-long drinking binges. She's involved in an incestuous and abusive relationship, but she doesn't have anywhere to go or anyone to help her. At 19, her future is set. She will most likely stay with her family, continuing to be both sexually and physically abused, until she marries and starts the cycle anew.
The idea of having an affair with a black man is exciting in a dangerous sort of way, but more importantly, making advances toward Tom gives Mayella power. This completely powerless woman has total control over Tom in this situation. If he were to agree to a liaison with her, then he would remain at her beck and call for the rest of his life. Readers know what happened when he didn't agree.
