Rashi: Now that Amasa died, Yo'av did not veer from his position. Via Yo'av, David needed to appoint his colleagues.

Radak: That was after Hashem saved him from all his wars, and Beis Sha'ul was finished off. Now he returned to his kingship after Avshalom and Sheva ben Bichri died, it again lists his officers, as if now his kingship begins.

Malbim: Also Yo'av killed Avshalom, David decided to make Amasa the head general in place of Yo'av, but Yo'av killed him. David then intended to put Benayahu in place of Yo'av, like Shlomo did after he killed Yo'av. However, after Yo'av killed Sheva ben Bichri, David saw that he cannot, for Yisrael lean to follow Yo'av - "Bnei Tzeruyah Kashim Mimeni" (3:39), so Yo'av remained the head general, and Benayahu remained over ha'Kereisi veha'Pleisi.

Malbim (24): People followed Avshalom, for he misled them to think that David will not judge their cases. They thought that the Mazkir and Sofer are not doing properly, to mention and write all who come to the king for Mishpat, and wanted to depose them. The verse teaches that they kept their positions, for the people realized that Avshalom lied.

Radak: It is because David wanted to make Amasa the head general in place of Yo'av, on the entire army or part of it. Now that Amasa died and Yo'av killed Sheva ben Bichri, who was Mored b'Malchus, Yo'av became haughty against David, David could not demote him. He appointed him over the entire army, and delayed [punishment for] Yo'av's sin] until after David's death (he commanded Shlomo to kill him).

Refer to 8:18:1:1-5 and the notes there.