Board Thread:Lore Discussion/@comment-29729442-20161212012142/@comment-26893431-20161212160216

Light armor is meant for scouts and skirmishers, light cavalry and archers. It's also possible that berserkers make use of said armor.

Also, the Legion can't afford to send a proper Legion and we know that they are forced to recruit locally, so of course they don't have their normal troops available. Heck, even Beirand says Light for scouting detail, Heavy for full-on combat, or something in between.