And just to expand on the differences between the SKS and the AK, the SKS was (is) a short-stroke gas piston operated semi-automatic rifle with a tilting block mechanism used to lock the bolt. The AK is a long-stroke gas piston operated select-fire rifle utilizing a rotating bolt.
The Garand is a long stroke gas piston operated semi-auto using a rotating bolt, so actually, the AK has more in common with the Garand than the SKS does, mechanically speaking. In fact, while I don't think it has ever been proven MK copied anything from the Stg-44, it has been well established that he did borrow heavily from the Garand in the operation of the Kalashnikov's trigger mechanism.
The SKS is a decent rifle, but it doesn't fit the Russian's military doctrine, which favors quantity over quality for pretty much everything. They came to appreciate the ability of massed fire to overcome shortcomings in individual weapons training by a poor conscript army. While clearing buildings and city streets, the Ppsh SMG was used extensively because its large capacity and automatic fire fit this doctrine. The Russians were looking for a similar concept from a rifle with a little more range and power. The SKS didn't provide this, and regardless of how it could have been modified to fit this role, the Russians felt it necessary to replace it. In the process, they gave us one hell of a truck/trunk gun though, and a pretty decent Ranch Rifle. I still don't understand how people pay Ruger's prices for a Mini-14/30 when an SKS is just as accurate, more reliable, and far more durable at 1/3 the price, but whatever...