Butler did say he was writing a book so maybe he is holding the nugget to sell the book?Because he obviously knows.
The fact he's never said anything should really be enough of an answer for everyone, but it's not. And Belichick isn't going to throw him under the bus.
Could also be the truth is going to make Butler AND BB look bad. Butler was wrong, BB was too strict and lost the SB, and neither will come out well with the truth. The other reason is it is so personal nobody wants it out there.
If Butler does not put it in the book, I think we will never hear the truth from BB or Butler to get 100% confirmation of what happened.