Aside from her personal reasons, there are two good reasons why not. First, it may not even qualify under the statute of limitations - it may not meet the definition of a crime with no limitation, and charging that would require prosecutorial discretion - and that leads to the second reason, that almost no prosecutors would touch this one.
A good reason not to press charges but also to come forward would be if she doesn't think he should be in prison in 2018 for that, but also should not be on the Supreme Court. It's not like this requires much brain power to work out.