Harris has to do what is best for the country and right now, she is best for the country.
Pete Buttigieg brings his own anti-baggage with him. He's a nice enough guy and a great spokesman but he has no foreign policy nor executive experience. Harris needs a VP that she doesn't have to raise from scratch during these trying times.
Maybe Shapiro's pro-Israel stance could be another plus for Harris. Send him over to talk some sense into Netanyahu. I know it's not that simple, but the choice of Shapiro could soften the harsh relationship between the U.S. and Israel.
No matter what Harris does, she going to make somebody or some anti-group unhappy. She'll do whatever is overall best for the U.S.A. and democracy.
"Why"
Probably because it's an appointed position ~ not elected ~ and not all FBI Directors are lucky enough to have a non-interference President like Joe Biden.
And there is always the danger of ending up like James Comey.