Because the entire story is that they didn't necessarily know what was going on? The one incident (in 2002 I think?) was reported to them, they cleared Sandusky because there wasn't enough evidence. No one ever reported anything to the police after that.
If you read the Grand Jury transcript, there are enough red flags that it seems impossible that no one put together the "big picture' until now. That tells me that there are a lot of people who were willing to look the other way.
The timing of the Sandusky's retirement also calls into question how much Paterno knew... considering that there was a campus investigation in 98 and shortly thereafter he was told by Paterno that he wouldn't be promoted to head coach... seems a little too coincidental to me.