I believe part of the answer comes from where the person hunts, and lives. I was raised in farming, ranch and logging country, and have spent many days in different Western States. Within the same section of land the topography changes along with the vegetation. The land mass in the country I was raised in exceeds the size of a cluster of eastern states, and the elevation tops out around 14,179 feet and bottoms around 1300 foot elevation.
The western county can receive between 70 to 100 inches of rainfall rapidly tapering off Eastward to around 12 inches of rainfall. Drought conditions are common on the Eastside. A very general statement could be made that these conditions also influence the selection of a new rifle among many other factors. The eastside offered some long range shooting, but broke volcanic terrain cover with lava rock, mountain mahogany, Manzanita, stands of juniper and mixed conifer balance the equation towards close range opportunities.
In general the blacktail deer lives on the Westside and the Mule deer on the Eastside. Black bear are common with the population being denser on the Westside.
During my youth the ½ ton pickup with a gun rack in the window was highly represented in the transportation category. It was a big event when Dad and I installed the 3-gun rack in the back window of his 1950 Chevy pickup. Early gun rack census would lean towards the Winchester Model 94 carbine with a twenty inch barrel in majority. Today it is a bolt action rifle, most likely a Remington 700, and there are more ¾ ton and 1 ton pickups.
Your late 1950’s and 1960 transition period is on the mark. I believe the greatest influence on barrel length and cartridge in the U.S. was the use of the 30-06 cartridge in WWII and Korean War by the U.S. military. The two primary U.S. military rifles of the period, the 1903 Springfield series, and the M1 Garand came with a 24-inch barrel. There were some 1917 Enfield rifles in 30-06, which was a modification of the British Pattern 14 rifle that had a 26-inch barrel. The 1917 was adopted as necessity during the WWI to fill a manufacturing need with existing tooling.
A successful military cartridge and rifle influences the populist. In my area the most popular military surplus rifle was the 1903 Springfield. A friend has a 1903 that he used when he was in high school. He shades a tear because he cut the barrel back to twenty inches.
I recall seeing a few long barrel deer rifles around in the 1950’s. These were the old 30-40 Krag military rifles that had been surpluses in the 1930’s. They were a cheap source of deer rifles. I do not recall seeing a commercial version. At the time they went into surplus the world was in a depression and knowing sportsmen with money knew there was a better cartridge and rifle to be had. I was a little guy watching Dad and a hunting partner skin out their bucks. The hunting partner had shot a buck with his long tom 30-40 just under the tailbone, and the bullet left a large exit hole in the chest. At that time I thought it had to be ultimate deer cartridge.
Unlike the British 303 that started life as a black powder cartridge chambered in the Lee Medford Mark 1 with a 30+ inch barrel, the 1903 cartridge was designed around smokeless powder which could achieve the required ballistics without a long tube.
I believe the nature of the hunter is to carry a light rifle, rather than a heavy rifle fitting for the caliber. The transition in America started slowly with the smokeless powder 30-30 Winchester. The lighter nickel steel barrel came into being to contain pressure and perform with the higher velocity round.
The continuing improvements in smokeless powder have and will influence barrel length.
Without a doubt the 30-06 set the gold standard in post WWII America. The cartridge has improved with newer powders and better bullets. The 30-06 is the 300 H&H of the 1930’s without the long barrel.
Gun writers the like of Jack O’Connor influenced my generation. Next to me I have a copy of his book, The Hunting Rifle,” and a recent GUNS magazine. His is also known for his extensive writing as the firearms editor of Outdoor Life. I looked forward every month to read his column and articles. I am sure that when he wrote about having his Model 70 Winchesters barrel turned down and cut back from 24 to 22-inches readers and manufactures took note. Did the Winchester Model 70 precede our follow his customizing.
I have spent a lot of time hunting thick brush fields on steep slopes. There are a few spots the blacktail bucks bed down that the only way to get to them is on the crawl. They are rather secure in these spots because most hunters drive by on the road and continue up the mountain. A short barrel rifle no manner what the action is better than a long barrel. Many times they are caught sleeping their bed. This type of hunting is tough on knees and elbow, and coming nose to nose with a rattlesnake is disconcerting.
Over the last sixty years the forest on the West Coast of the United States and Canada have changed do to looking. What was thick forest became clearcut blocks. When the forest was opened up vegetation (deer food) was abundant and the deer herd increase. The distance in which deer could be spotted and taken also increased from less than fifty yards to hundreds of yards. What had been 30-30 country became 30-06 and 270 country.
A few years ago I was looking around for a 270 Winchester with a 24-inch barrel, the purpose to gain a little more velocity without upping the load. At the time I could not find one on the shelve. More recently there were a number of them available. Some of these rifles did not gain weight with the extra two inches, because of slimmer barrels, fluting of the barrel, and better stocks. Then my game plan changed because I had run across some take-off barrels for the Remington 700 in 24-inch length at a gunshow. I put off buying expecting to take advantage on another day. I changed my mind about that after killing a couple of bucks with the rifle and 22-inch barrel. It was creating more than enough damage with the 22-inch barrel.
At one time I had a Husqvarna Hi-Power in 30-06 with a23 ¾ barrel in 30-06. It had a nicely blued barrel and jeweled bolt. A smooth, reliable rifle. The downside to the rifle was a rather hefty birch stock.
I was brought up shooting doves, quail, pheasant, grouse, ducks and geese with a 30-inch double. Now days most of my bird hunting centers on quail, grouse, and pigeons. My GoTo shotgun and a 28-inch modified barrel Ithaca featherweight pump. After a successful week shooting mountain quail and grouse I was hurting. I installed a thick recoil pad on it.
The 30-inch double was great for pass shooting waterfowl, but as an anxious teenage I missed a lot of birds jump shooting them out of the river or irrigation ditches at close range. Rather tell tale on the south end of a Millard centered at 20-yards.
Most of the large sporting arms companies in the United States have been tied to the production of military small arms. It is logical that the tooling used for military production turned to the sporting arms at the end of the big contracts. As an example the 1917 Enfield/Pattern 14 manufacture by Remington became the Remington Model 30 sporting rifle. Winchester took a different path using the 1903 Springfield and tooling.
Teddy Roosevelt wrote about his African adventure using a couple of 1903 Springfield built at the Springfield Armory. Yep at one time we had a real president.
Logic has it that if you have equipment on hand to build rifles to a government spec that if this can be translated to the civilian market, cost per unit goes down.
My logic easily subject to challenge is the same thing happen in Britain. After WWI and WWII arms factory looked for another source of income. They were designed and equipped to build long barrel rifles. Rather than created something new they modified their production to fill the sporting needs. The custom makes may have utilized some of the tools of military production where suited.