It's been said on some of these threads before, but the reason the 30-06 is not away ahead of the 270 and 308 in popularity, is because of the loadings for the 30-06. Old faithful has been around for over 100 years, it has been manufactured in every country that has made sporting rifles and it has been made in every action anyone has ever thought of. Prior to WW1, steel was sometimes hardened a bit haphazardly. For example, the early military rifles made at the Springfield Armoury, had their bolts heat trated by the color of the hot steel! Thus, they now admit that bolts tempered on a bright day, or at better light conditions around their equipment, would be hotter, and thus made more brittle, than would bolts tempered when it was duller, and the steel would appear to be hotter than it actually was. I am sure we have all heard about the low serial number Springfields that may have weak actions, and there are some of them still out there! Also, the Springields were designed for 45,000 psi pressure.
With all the vast array of potentially weak 30-06 rifles out there, the loading companies, both factory loadings and loadings given in manuals, make sure the pressures are down enough to accomidate a weaker action.
The 270 and 308, on the other hand, came out after making steel was a science and there are no weak actions out there for either calibre. Thus, they were loaded to about 55,000 psi.
A modern, bolt action 30-06 can easily be loaded to give a 180 grain bullet 200 feet per second more velocity than the same bullet in a 308. If this figure was with factory, or loading manual loads, and the public knew this, the 30-06 would be head and shoulders above the other two in popularity.
About twenty years ago I started to chronograph loads. I was amazed to find that the 180 grain bullets in the 30-06, either factory loads or loads from a manual, which all claimed 2700 fps, were actually leaving the barrel about 2550 feet per second and sometimes less than that!
I also remember very well when the 308 came out. The gun writers in the glossy, vast circulation American shooting magazines, wrote that handloaders would have a hard time with the 308, in even equalling factory loaded velocities. They said this was because factory loads were right up to full pressure, and a handloader would maybe only have one powder available that would allow him to equal factory velocities.