5.1.3 ........Wing, or squadron commanders may prescribe unit designation emblems, not to exceed 5 inches in diameter, in cloth or silk screen, worn on the left side of chest of both the desert sand and squadron commander-prescribed Friday morale undershirts. In-theater commander will determine if Airmen are authorized to wear standardized color undershirts with or without unit designation emblems. Neither unit designation emblems or commercial logos will be visible when wearing the ABU coat (shirt). .........
I would assume this is the reason.