I can.
1.) The kid wasn't just any kid. Both Sandusky and an adult AM (until he was paid $ 7 million dollars to change his story) considered themselves to be father and son. Should Sandusky not have been allowed to shower with one of his adopted sons? Are you ashamed to shower with your own blood son?
2.) Having possible hypogonadism, in HIS mind, it was nothing more than your typical shower after a workout.
3.) Sandusky wasn't the brightest lightbulb in the world when it came to common sense and how the world was changing. I took lots of showers at Rec Hall and the new (for me) IM Building near the Stadium late into the night (after 9 or 10 o'clock). So did lots of other guys. But none since the Sandusky incident exploded in 2011. For the life of me, I seriously can't remember if a kid was ever in the shower with another guy during all those years or not, because if he was, prior to November 2011, it wouldn't have meant a thing to me.
4.) When I was growing up, you were considered to be weird NOT to want to shower in front of other guys. You'd be considered "queer". Maybe Sandusky still carried that mentality in 2000/2001.
5.) The way Lauro consoled Sandusky and assured him by phone and mail that the 1998 charges were "unfounded", it may have given Sandusky some form of redemption. Remember. He was never even aware an investigation was going on until Lauro and Scheffler questioned him in the locker room. Lauro did most of the talking and pretty much downplayed the whole incident.
You asked for some reasons. I gave you some. Sometimes it takes time for us to accept change. Maybe 2000/2001 was that "change" for Sandusky. After the 2001 "swim suit" talk with Raykovitz and the talk with Curley, as far as I know, Sandusky never showered with a kid in a Penn State locker room again. ("Victim" 5's original claim was 1996-1998).