It's all about cylinder size. With six holes, the .480 doesn't have a huge amount of steel in between holes. Even higher pressure probably isn't a good idea. And the longer cartridge of the .475 would be problematic as well. If they were going that direction, they would probably have to come out with a longer cylinder version. A Ruger X frame.
They did make the decision before to make the .480 SRH a five shooter... then they only made about 50 of them (all Alaskans) and quit. When I heard they were going to make .480's again, I assumed they would be five shot. I was wrong!