I think the choice was a combination of a couple of reasons. They likely chose the round because of similar experiences to Afghanistan where the 5.56mm rounds proved incapable of use in a lot of combat situations. Not enough stopping power. Nor enough range. The intermediate Soviet round of the AK-47 solves the stopping power issue. The bullet is larger. As for the weapon proper it was likely a matter of who could setup a factory which could produce millions of rifles on the cheap. Israeli weapons like the Galil ACE might be nice and all but the Israeli's don't produce weapons in those sorts of numbers. Never did and likely never will. AFAIK the major client of the Galil ACE is Vietnam. Which AFAIK also use the 7.62x39mm round.