Abner spoke to the Benjamites in person because they were from his tribe and he was asking them to reject one of their own (Ish-bosheth) in order to accept David.1 He thus needed to be more persuasive and ready to answer any objections which they might have had. This is something better done in person than through a letter.
18 établissez-le maintenant, car l'Eternel a dit de lui: C'est par David, mon serviteur, que je délivrerai mon peuple d'Israël de la main des Philistins et de la main de tous ses ennemis.