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 Now then bring it about, for the LORD has promised David, saying, ‘By the hand of my servant David I will save my people Israel from the hand of the Philistines, and from the hand of all their enemies.’”