Because atheists who attack the authenticity of the Scriptures point out that Herod did not rule in 1 A.D. and use this to claim that the Gospels are wrong.
The Pope is pointing out that it was a calendarist who miscalculated, and not that the Evangelists lied.
I see absolutely nothing wrong with taking an honest and frank review of the history of one’s religion. to confirm that it can withstand critical review.
After all, Mormons do this all the time!
/s