Indy,
Because 6.5mm and 7mm originated as military calibers back in the early days of smokeless powder, when all military bullets were long, heavy roundnoses, apparently because all previous lead bullets for black powder military cartridges were long, heavy roundnoses. As a result, relatively fast twists became standard in 6.5 and 7mm, which also turned out to work fine to stabilize long, streamlined spitzers when they started appearing.
The .270 Winchester appeared after the transition to spitzers, when high muzzle velocity became the obsession among hunters. Many companies, however, found that faster twists resulted in poor accuracy. Early spitzers were often poorly balanced, and faster twists accentuated the poor balance.
As a result, companies introducing new, high velocity hunting cartridges often used relatively slow twists that would JUST stabilize lighter bullets. A classic example is the .250-3000 with its 1-14 twist, but the .270 was also one.
It's standard 1-10 twist is considerably slower than the twists normally used in 6.5 and 7mm calibers on either side of .270. Which is why heavier 6.5mm and 7mm spitzers have higher BC's than .270 bullets: Their standard twists can handle longer, more streamlined spitzers.
Consequently, when the long-range hunting trend started after the introduction of laser rangefinders, hunters "discovered" what target shooters had known for decades: 6.5mm and 7mm bullets were available with higher BC's than .270 bullets, and hence worked better at longer ranges.
Yeah, you can put faster twists in .270-caliber rifles, and there are a few high-BC .270 bullets designed to work in them. But why bother after decades of 6.5mm and 7mm twists and bullets providing better long-range performance?
Yes, the .270 was already "proven." It was proven to result in less retained velocity and more wind-drift at longer ranges than 6.5mm and 7mm bullets--which were not a new fad. They had been proven for even longer.
Not everyone went "all goo goo ga ga over the 7mm and later the 6.5mm", just hunters who were ignorant of their long-range advantages, which target shooters had known about for a LONG time. Which is exactly why .270 caliber never became popular among target shooters.
