To omit the Levites would be to follow the precedent recorded in Numbers 1:47-49. The omission of Benjamin must he ascribed to a determination on the part of Joab to frustrate the king’s intention, whereby he might hope to avert God’s wrath from the people.
Related Commentaries of 1 Chronicles 21:6
1 Chronicles 21:6
6 But Levi and Benjamin counted he not among them: for the king's word was abominable to Joab.