I think it' s because the 308 doesn' t have the case capacity to EFFICIENTLY shoot the heavier bullets... 180 and above. It CAN shoot them, but it' s just not quite as efficient from an over-the-counter hunting cartridge standpoint. An astute handloader that realizes that velocity isn' t quite the whole story can reload 180' s or 200' s in a 308 and have a great load that penetrates VERY well....... IF the twist in his barrel is fast enough to stabilize those longer than normal bullets.
The 30-06 has the powder capacity to propel those heavier bullets at " hunting" velocities, so it has a faster twist barrel to allow that. ...... Just my opinion....