I honestly don't know why, other than just because... no good reason
I have a 30 carbine blackhawk, & will keep it... it's actually less powerful & less versitile than the 357's... I think when they came out, it was to take advantage of all the cheap surpluss ammo... that is gone now, so buying cheap new manufacture, is no different than buying cheap 357 ammo... the carbine, is very marginal on deer anything bigger than the dog sized varieties put the cartridge in a 6" barrel, & it's a wood chuck to yote gun at best, & offers nothing that can't be done better with the 357 or even a 38...
that said, handloading with faster powders makes it better in the revolver, but not better than the 357...
I'm up in the air right now, about doing a custom on my 30 carbine blackhawk, or keeping it as such... I've had it since those days of cheap surplus, & I have a carbine & an Automag 3 that shoot the cartridge, so I like it... just don't think it's going to warrent a new model