When it came about weapons design was at a crossroads. The US had the Garand and Johnson, Germans were expirimenting with semi auto full size cartrige designs. The russians had the SVT38/40. Most of the long arms in existence on the battlefield were bolt guns, and a semi auto rifle was a step ahead in evolution, however the germans were also due to war experience melding the SMG and rifle into one concept (the assault rifle) and weapons design was taking another road of parallel design. The SKS fit into the semi-auto rifle category, and in that it would have excelled if it had been introduced earlier, and the fact it used an intermediate cartridge arguably put it a step ahead. The AK being shaped off the Germans assault rifle concept overshadowed it, not because the SKS was a failure, but the AK fit russian frontline tactics better, and manufacturing was simpler so that you could crank out more rifles for less cash and time.