the cartridges design parameters differ simply because the way each was intender to be used differs,
the 308 win cartridge was designed to be used in a
full size battle rifle (M14) with the idea precise rapid accurate semi auto shots would be made at longer ranges,
each individual combatant was expected to pick out and engage individual targets rapidly and eliminate the threat,
basically an improved and upgraded M1 GARAND with a detachable 20 cartridge capacity battle rifle with the option to be used with full automatic fire capability, but it takes considerable practice to fire short accurate bursts and maintain hits past 75-100 yards.
shortly after WWII, the concept of each man is intended to be a skilled rifleman, was still the predominate US ARMY/MARINE concept.
the 7.62/39 was a totally different concept from its inception, from the onset the soviet idea was to equip large basically marginally trained personnel with a weapon designed to provide full auto suppressive fire ,with reduced recoil, and targets past about 350 yards were not really of concern,
and a weapon that could be handed out to people who might be far less inclined to provide routine maintenance at regular intervals.
they liked the concepted they saw in the nazi STG44, and respected its capability
soviet doctrine assumed most fire fights would be in areas like cities or places where shots past 350 yards would be handled with machine guns like the PK,RPD, or RPK, all larger machineguns chambered in
7.62×54mmR or artillery or tanks would be used to engage the threats. remember soviets equipped whole battalion's with the PPSH 41 pistol caliber carbines in WWII,
the soviets designed individual combatants weapons to provide a concentrated hail of projectiles but most of the combatants were not expected to be snipers, who are trained to provide longer range accurate fire,
thus masses of durable carbines with high mag capacity , necessitating a lower recoil cartridge with a bit less range to reduce recoil was designed.