..........I don't think there is anything special about the 6.5mm/.264 caliber. What is special is the advancement in bullet technology.
.........When all we had were soft-point, non-bonded bullets, the .308 caliber made sense, because with the right weight you could overcome deficiencies (of the bullets) and get necessary penetration for effective kills.
.......Then we got partitions. Then bonded. Then monolithics. Manufacturers mastered controlled expansion across a broader range of velocities.
.......The result is that now we can do the same things we used to do with .308 bullets with smaller and smaller calibers. It's similar to the FBI moving from the .40 caliber to the 9mm. The .40 caliber can still do the job, but now the the smaller caliber can tick all the boxes. That's because of bullet technology, not because there's anything special about the 9mm handgun caliber.
........6.5mm bullets can do more today than ever before, as can bullets of every other caliber for the same reasons. But when several calibers will work, why not go with the smallest, softest-shooting choice? Companies like Hornady have capitalized on this trend wisely. But they didn't create it..........