Because PF knew how to manipulate KB.
He used all the tricks abusers use.
Started out being the strong, silent cowboy in their relationship, which then evolved to putting KB down all the time, controlling every move she made and no matter how hard she tried, he kept putting her down and belittling her.
He was a crazy-maker. KB was a religious, perhaps slightly subservient woman and believed that a woman should be "obedient" to a man, but PF's actions may have caused her to doubt that and rebel, taking baby K with her.
Abusers like him follow a pattern. Eventually the abuse victim (KB) believes that she's the one who is wrong and will do everything they can to mollify and try to keep the abuser's affections. But its never good enough for the abuser. The harder you try the more they abuse and derive delight and power from it.
Its pure evil but it happens all the time.
And it happens to strong, intelligent, independent, self-aware women.
These monsters have the knack of seducing, abusing, controlling and destroying, and its not all that hard for a decent person to fall prey to.
moo