Prosecutors declining to charge isn't the same as exoneration, as much as a defense lawyer might want to convince you of such.
There are plenty of crimes that don't leave particularly useful evidence. Pointing a gun at someone is illegal, but how do you prove it happened? Same issue in many rape cases.
Cops in some areas also have a history of suppressing reporting and/or evidence to keep crime statistics down. http://en.wikipedia.org/wiki/Adrian_Schoolcraft
Particularly relevant is the fact that only 14-18% of sexual assault cases are prosecuted, or 37% for rape alone. http://www.uky.edu/CRVAW/files/TopTen/07_Rape_Prosecution.pd...
Given that, it would actually be highly unusual if he DID go to trial.
Wanted to ask you for a link, went on and found one myself:
http://ia701204.us.archive.org/19/items/gov.uscourts.ord.115...