As to why SKS versus AK, you have to remember that the Soviet designers under Stalin were forced to be very competitive with each other. Simonov, who design the SKS, designed the AVS-36 (Avtomatic Vintova Simonova) which failed in action. Simonov corrected the problems with the AVS but was beaten out by Tokarev's SVT-38 and the later SVT-40 (rumor is that Stalin liked Tokarev better than Simonov, but who is to say?). After the war, Simonov, Tokarev, and Kalashnikov all competed in designing the new rifle post war. Simonov's design was accepted as it had been designed during the war, more or less, and is called by us SKS-45 on occaision. (Kalashnikov wasn't finished) and placed into service. The AK was not evolved from the SKS in the least, and beyond some similar contours in the gas system plus the common round are unrelated. The AK was clearly the better weapon.
The SKS was continued in production for reasons I cannot say other than as a hedge against the AK and, with tooling and facilities in place, producing a rifle that can be used for guards or other second line troops was a possible consideration.
And, you can use the SKS to tell which nations were more friendly to the Soviets than others. Those nations which were supported by and were more supportive of the Soviets, or were considered more stratiegic against the West recieved AK designs (Poland, Hungary, Bulgaria) whereas the nations that were not so friendly but not aligned against the Soviets, such as China, Romania, Yugoslavia, and Albania, went the SKS route. This provided weapons to fight against the west without providing weapons that were of the same combat capability. The Yugos kept the SKS design longer because the SKS was better at launching rifle grenades. The Albanians were late adopting the SKS and never used it front line. However, the Romanians and Albanians were Maoist in their brand of communism and Tito was a bit of an Iron Curtain maverick in Yugoslavia.
In the 1970's distrubiting AK's as foreign aid became quite popular for the Soviets (especially since they had adopted a new rifle, the 74, and a gazillion .30 caliber Ak's about).
Who can explain the Chinese? They issued SKS's, AK's, and Mosin Nagants at the same time. They call the AK a rifle and the SKS a carbine, when the AK is clearly shorter than the SKS (I know, I know, their rifles are all Type rifles of some kind). Many SKS's were made for export, and many were made for use by the peasant army while the AK's were kept for front line service.
Ash