The answer most probably lies with the difference in the Breedlove and Macteer families. While the Macteer family is poor, and the girls often feel that their parents treat them harshly, there is no question that all members of the family love and care for one another. Claudia describes how her mother cared for the girls when they were sick, and the entire family rallies around Frieda when she is molested.
Contrast that with the Breedlove family. Pecola's entire life is dominated by violence, shouting, and hostile silence. Her mother beats her when she drops a pie on the floor of her employers' home, and she beats her when she finds out she's been raped. There is no semblance of love or even sympathy within the family. In fact, while Frieda's molester is a stranger, Pecola's rapist is her own father. Without the strong support of a family to guide her through her traumatizing experience, Pecola is left to deal with her pain on her own. Rather than coming out stronger, she collapses, eventually losing her mind.