Howdy
The 30/30 round was the first commercial Smokeless rifle round manufactured in the US. The 30-40 Krag is a little bit older, but it was a military round. The 30/30 was first developed for the Winchester Model 1894, although I believe the round was not actually commercially available until 1895. So that does put in the correct era for Cowboy shooting, albeit it at the tail end. The 30/30 was originally developed as a high velocity smokeless rifle round to take the place of older, slower Black Powder rifle rounds with rainbow trajectories like the 45-70. Not that the 30/30 is as powerful as the 45-70, but it is much flatter shooting.
The reason the 30/30 is not acceptable for SASS as a Main Match rifle round is that it is not a 'pistol' round, it is a longer, bottle necked rifle round, no conventional revolver could chamber it, and it is generally too powerful and would damage targets.