The Lewinsky scandal gave Hillary huge approval ratings. The American people felt bad for her and she gained a huge amount of sympathy.
Dragging that out again, especially when so many voters don't care or don't remember it, would just hurt Trump.
2016 is a lot differently culturally from 1997. Had the scandal happened today, for instance, it's basically impossible for me to imagine she stays with him. It would be much more politically prudent for her to leave. By the same token, I can't imagine the sympathy factory would play out the same. I think people would view her pitiful or foolish more than anything.
