What a lot of people don't realize is that when you specify a particular cartridge, such as the .270, then, in addition to a certain range of bullet weights and muzzle velocities, you are also specifying chamber dimensions and twist rate, all based on SAMMI specs decided at the time of cartridge acceptance.
Preferred chamber dimensions/twist rates have evolved over the last decade or two towards longer throats and faster twists as long range shooting has become all the rage. However, the SAMMI specs on the older cartridges do not change. Consequently, anything but a custom barrel will have the "old fashion" specs, and thus not interest the long range crowd. That old cartridge then falls out of favor with the young and hip, even though it is as good as it ever was. The young folks are the ones buying the rifles, which then makes the industry tend to migrate away from the older classics.
The .270 bore, always having been an outlier bore diameter, is even more susceptible to this inexorable shift in cartridge/shooting preferences.
