Adam Clarke on 1 Chronicles 21:6

Levi and Benjamin counted he not - The rabbins give the following reason for this: Joab, seeing that this would bring down destruction upon the people, purposed to save two tribes. Should David ask, Why have you not numbered the Levites? Joab purposed to say, Because the Levites are not reckoned among the children of Israel. Should he ask, Why have you not numbered Benjamin? he would answer, Benjamin has been already sufficiently punished, on account of the treatment of the woman at Gibeah: if, therefore, this tribe were to be again punished, who would remain?