Why didn't they bring out a version in 9mm Luger instead?
They did: The Colt Commander.
After WWII the US Army sponsored tests for a lighter handgun supposedly for officers. The spec included requirements for a max length of 7" and a max weight of 25 oz. Colt submitted what eventually became known as the Colt Commander in 9mm built on an aluminum "Coltalloy" frame. The Army didn't follow through with selecting anything, but Colt introduced the pistol to the public in 9mm, .45 ACP, and .38 Super.
