The short answer is that the technology didn't exist. Ballistic shields are made using modern polymers which hadn't been developed in the 1940s. The first ballistic shield I ever saw was in 1993.
Also, a ballistic shield that will stop rifle rounds is HEAVY. A level IV shield the same size as the shield in the picture weighs close to 65 pounds and comes with wheels.
There's a reason you don't typically see any military deploying them.