Minnie recognizes the challenges of telling the truth on both social and personal levels. Given the social unrest and the intimidation that many Whites were perpetrating against people of color, Minnie understands that if she speaks out in the form of the book, reprisal is almost guaranteed. As a woman of color, she understands that her lack of social power is one in which there is huge risk in speaking out and participating in the book. Given her children and their need for her, she has more to lose than Aibileen.
Another reason why the truth about working for her employers weighs so heavily is because of the pie she baked for Miss Hilly. That act of resistance was so brazen and so defiant that she won't even refer to it by name. She recognizes that what she did crossed so many boundaries that telling the truth and disclosing it is something that she could not bring herself to do. Minnie comes around after reflection. Yet, the opportunity to "tell the truth" is not something she immediately embraces because of what the costs are on both personal and spiritual levels. It is here in which one sees how the weight bears down on her initially.