Possibilities:

1) He isn't as intelligent as advertised.
2) He wanted to write a book for yet more money and thought that he would have political protection from a party he was certain would retain power.
3) He is a megalomaniac with delusions of grandeur who saw the diary as a necessity for historical posterity - an inside records of the travails of a historical giant grappling with the hard questions only few could ever shoulder.
4) He never thought anyone would be able to obtain it before he died ( see #1 and #3 again).

Anthony Fauci has caused more harm to people than any doctor in the history of the world, and it is not even close. A malignant narcissist by definition. I hope that he seeks forgiveness (as we all need to do in our lives).