One of the Soviets' main concerns was to insure that production was not impaired. In late 1941, when much of their heavy industry was moved to the Urals, it took 3-6 months before production was even approaching prewar levels. This was a very difficult time, because it meant that replacements from fall of 1941 to spring of 1942 were very sparse. Once production stabilized, it became of utmost importance that production be maintained and, if possible, increased. In fact, this was one area that Stalin was very, very concerned about.
Thus, when any prototype was posed before the Red Army for approval, one very big factor was how much would it impact on present production (retooling of machinery, etc.), and in such an event would it's combat effectiveness be worth the disruption to production? When it came to the question of manufacturing an armoured personel carrier, the answer was 'no', because it would have impacted on tank production. This was the same reason trucks were lend leased, rather than built.
