Albert Barnes Commentary 1 Chronicles 12:29

"And of the children of Benjamin, the brethren of Saul, three thousand: for hitherto the greatest part of them had kept their allegiance to the house of Saul." — 1 Chronicles 12:29 (ASV)

For hitherto... — Rather, “For still the greatest part of them maintained their allegiance to the house of Saul.” This is given as the reason for so few coming to Hebron. It shows us that, even after the death of Ishbosheth, the Benjamites had hopes of furnishing a third king to the nation.