The reason the 170 gr 30-30 bullet works so well on game lies in the ease of designing a bullet to properly expand over its expected velocity range.

For example, in 30-30 there is only ~400 fps difference in its muzzle velocity and its velocity at its maximum useful range. In 30-06, that difference is ~1100 fps.

It is much easier to make a copper jacketed, lead core bullet expand properly with a ~400 fps velocity window vs an ~1100 fps velocity window. Think about it.