When I was a boy on the ranch, I carried a .30-30 on the saddle constantly -- I killed plenty of deer, and sometimes coyotes, coons, groundhogs, whatever I ran into. The rifle was a tool.
The rifle is no longer an always-with-me tool, it's a part of a special dream -- the annual elk hunt. The hunt is special, and so must the rifle be (I hope we all realize the rifle is a sacred instrument.) I could kill an elk with my old .30-30 -- but just being in the mountains with Bigfoot Wallace, my custom '03 in .35 Brown-Whelen means something. I see a guy with a shiny new .300 RUM, and I don't laugh at him -- I understand him.
Also, I think part of the problem is we assume the old timers used what was best -- in reality they used what was available. For a long time, for example, there were precious few bolt-action rifles made in America, other than military. Even right up until just before the war, finding a magnum-length action was difficult. But .30-30s were plentiful. And bubbaized Springfields, M1917 Enfields and Mausers were cheap.
Another part of the problem is that the game has indeed changed. Elk were once found throughout the continent, mostly in low-lying areas. People lived on farms, and had plenty of opportunities to hunt. Nowadays, while game is once again plentiful, it isn't necessarily as readily accessable as it once was. When a man spends a whole year dreaming about one shot -- and drives half-way across the country to make it -- I can understand why he would want a special rifle.