Because the rifles have not been made in a while. I have been looking for a big bore for a couple of years and have found nothing but junk for premium prices.
There is nothing wrong with either of those cartridges. I have a .375 in an Encore and really like it.
It's the same old thing with cartrdges, some make it and some fall by the wayside. It doesn't mean that they are bad it's usually other reasons.
It will be interesting to see how many of the new "super mags" and other tricks of the year are around in ten years. The STW's are already starting to feel the pince from the factory mags.
I bought a Marlin Cowboy in 38/55,which is pretty close in size as the .375. It's fun to shoot and the gun is strong enough to reload to the .375 pressures if need be.
It's a new gun and I didn't pay an arm and a leg for it.