Why does the opportunity to tell truth about working for white people weight so heavily on Minny?

Minny is afraid that if she cooperates and tells her story to Skeeter for use in her book, she will lose her job and be unable to find another. She sees it as biting the hand that feeds her. Minny needs a job..... it's difficult for her to become a part of something that might just put her in a position from which she cannot extricate herself.

Minnie is also apprehensive about the fact that she knows secrets about her employer that would be inappropriate to devulge.... a moral concern.