Because like most people who didn't go to Penn State he's judged the man on the incomplete details presented in the media. Primarily the narrative that he didn't report Sandusky to the police. The problem with that narrative though, is that he did report the allegations to the campus police and administration. That part is commonly, but not universally reported. This is typically followed by people asking why he didn't tell the real police. The thing is, and what alost now news story has mentioned because no one does research anymore, is that the Penn State Campus Police at the main campus are the real police for the campus. Also, Sandusky was retired, he was on campus because the administrtion let him maintain an office and use their facilities for his, until this broke, highly regarded children's charity.
Consider this scenario:
A young man, who you've know for a few years and first met as a snot nosed freshman comes to you and (accounts vary, but let's go with the worst case) tells you that he saw a man who has been a good friend of yours for decades (longer then the young man's been alive) and is highly respected in the community raping a child in the shower.
What's your first reaction?
You then, despite your understandable doubts and emotional rejection of the very idea, report it to the police and bosses (since it happened at work). They take the report and then nothing happens.
Is it reasonable for you to come to the conclusion that the authorities investigated it and found nothing?
The board fired Joe because they hoped it would make them look better and because there's a good chunk of it that's wanted him gone for years because he doesn't commercialize the football program enough.
Note that while the men Paterno reported the incident too are being proscecuted, Paterno wasn't.