On the surface the .454 in a carbine sounds great. The limiting factor would of course be the pressure the .454 generates.
After thinking more on it though, it would seem to me the .454 would make a somewhat poor rifle round. Too powerful for the traditional pistol caliber rifles, and a weak sister compared to a "real rifle" round. The way I see it, is if I wanted, say a Marlin Guide Gun, which is pretty much what you would need to contain the pressure of a .454, why would I want that when I could have a 45/70? Honestly, I would still like a .454 in a Guide Gun, simply because it's different. For praticality though, the 45/70 makes alot more sense.
Just my thoughts as to why it isn't done.YMMV.