De Nada...
Now - understand, this is my take on it and nothing more - and someone far more knowledgeable in modern metallurgy might chime in.
I think Colt doesn't case-harden hammers for a couple of reasons:
A 'signature' of the 'modern' Colt Single Action Army is the polished hammer flat - and they seem to like that, plus, the polished flats are easier and cheaper to finish.
Casehardening is just another step in the process, but one that would require more handling and more cost - and Colts are already expensive, so they're trying to stay competitive - and Colt still forges the frame and backstrap and triggerguard, and 'that' is an expensive process, when other clone makers aren't.
Casehardening is 'microns' deep - no brittleness should occur with modern metals - 'but' could occur with older ones, way back when, and causing warpage and replacement - meaning a loss to the bottom line.
Your best option is to find someone who'll do the work - as I did - and fortunately, there are some folks stepping forward to do the work reasonably.
Vaya,
Scouts Out!