The clean legged birds in the states were originally imports from Britain, the only country with a clean legged marans standard.
Some of these were likely before the US adopted the French type, making these English types a bit redundant.
The thing is, since the US marans club has not yet recognised the cuckoo variety, the English type is no less correct, as they can’t be SOP anyway.
Since egg production is often higher in English type marans, they’ve managed to remain in the states, particularly in hatcheries where the success of a bloodline of often determined simply by its ability to reproduce (if hatcheries are hatching all eggs, those laying more will have more offspring).
But these birds now, as other have said, are long separated from imported birds, they have been American chickens for many generations.
If you want to breed SOP marans in the states, cuckoos simply aren’t the correct option, but they are still fanatic birds laying brilliant eggs.
I hope this makes sense.