Matthew Poole and continuators on 1 Chronicles 21:6

6 But Levi and Benjamin counted he not among them ; for the kings word was abominable to Joab.

Partly for the following Reason; and principally by Gods special and gracious Providence to these two Tribes, to Levi, because they were devoted to his Service; and to Benjamin, because they were the least of all the Tribes having been almost extinct, Iudg. 21. and because God foresaw that they would be faithful to the House of David in the division of the Tribes, and therefore he would not have them diminished. And Ioab presumed to leave these two Tribes unnumbred, because he had specious Pretences for it; for Levi, because they were no Warriours, and the Kings Command reached onely to those that drew sword, as appears from v. 5. And for Benjamin, because they, being so small a Tribe and bordering upon Ierusalem their Chief City, might easily be numbred afterward.