Because if you want to shoot Black Powder or reduced loads for Cowboy Action Shooting, the 45LC or any other cartridge that size is a real handful.
You can download smokeless powder for less recoil, but then the pressure is so low the brass case doesn't seal the chamber and blowback and powder residue getting into the action is a concern.
You can't effectively download Black Powder in a revolver. You should never have an airgap in the case and large wads in a revolver could cause issue in the forcing cone.
And a 45LC with case full of BP is just not competitive because of recoil in the hands of most CAS shooters.
A much better option than 45ACP in a revolver is the 45 Cowboy Special brass. A short version of the 45LC.
PERFECT for black powder in larger period correct revolvers. Plenty of smoke and boom, but much less recoil and cost of reloading.
And you simply use 45ACP dies to load for it. Simple!