From Chevy Wiki
"Bruin" followed the pattern of "frontier beast" names given to heavier Chevrolet trucks such as the Chevrolet Bison (heavier) and Chevrolet Kodiak (lighter). "Brigadier" followed the pattern of "military rank" names given to GMC's heavier trucks. Unlike the Titan/Astro & Bison/General, the Bruin/Brigadier found suitable replacements in the form of the Kodiak/Topkick
For some reason, the GMC version was more common than the Chevrolet version.