On Scandal author Ari Adut has some ideas as to why this might be.

Palin seems too new and popular at the moment to suffer from a scandal. As a woman, her “private” life and family is relatively more a part of her public persona than it is the case with male politicians. This allows for more potentially scandalous stuff to become public and discussed in the media.

But, at the same time, she is a very astute politician, and her private life (especially her motherhood and the challenges that it involves etc.) has become a big part of her charm – not for everybody of course but for a lot of Republicans and Independents and especially for a lot of middle-class women. So she seems, for the moment, immune. I am sure that with time she will become more vulnerable.