Albert Barnes Commentary 1 Chronicles 27:24

"Joab the son of Zeruiah began to number, but finished not; and there came wrath for this upon Israel; neither was the number put into the account in the chronicles of king David." — 1 Chronicles 27:24 (ASV)

Because there fell wrath - This literally means, “And there fell wrath.” The outpouring of God’s wrath was not the reason Joab stopped. His motive is clearly stated in 1 Chronicles 21:6. See also the cross-references.

Neither was the number… - This means that the final count of the people taken by Joab was not entered into the official chronicles of King David. That section of the chronicles recorded other numbers, such as the size of the standing army, the Levitical and priestly divisions, and the singers. The disastrous events that followed the census likely created a fear that God might be further provoked if the number was officially recorded in the state archives. Therefore, the numbers that have been passed down to us must have come from private sources.