Alma 48:5

and thus he did appoint Chief Captains of the Zoramites, they being the most acquainted with the strength of the Nephites, and their places of resort, and the weakest parts of their cities; therefore he appointed them to be Chief Captains over his armies.

He chose Zoramite leaders to be his top captains. They knew the most about the Nephites’ strength. They knew where the Nephites went. They knew the weak parts of their cities. So he made them the leaders of his armies.

He made the Zoramites his chief captains since they knew the Nephites’ military strength, where they gathered, and the weak points in their cities.