Albert Barnes Commentary 1 Chronicles 21:6

"But Levi and Benjamin counted he not among them; for the king`s word was abominable to Joab." — 1 Chronicles 21:6 (ASV)

To omit the Levites would be to follow the precedent recorded in Numbers 1:47-49. The omission of Benjamin must be ascribed to Joab's determination to frustrate the king’s intention, through which he might hope to avert God’s wrath from the people.