In the case of the AK74 the Soviets thought they were missing the on micro-calibers and wanted to look just as advanced as the US, plus with their tactical doctrine the reduction of strategic materials was important. Colonel Kalashnikov HATED the idea, fought against it, and told all and sundry that they needed to improve the 7.62mm rather than go smaller, he was overruled.
As far as the other countries, the early adopters were either getting huge quantities of ammo from us, or our allies (like Israel and South Africa) or they saw the writing on the wall about a forced adoption by NATO. America has always supplied the lions share of ammunition and equipment to NATO and they knew that we would eventually make them adopt the 5.56 as we did in 1979-80.
Personally I think that if we had adopted something like the British .280 or the 7x49mm instead of the 7.62x51 NATO, we might well be using it today. Instead we demanded the 7.62 NATO, offered to adopt the FN-FAL. We reneged on that deal and adopted the M-14, played political games and lied to congress and the SecDef about the cost and production numbers on the '14 and generally screwed the pooch until McNamara slapped the whole system and made them take the M-16.
The ballistics of the .280 are strangely close to the 6.8mm Rem with a 140gr 7mm bullet fired at 2,415fps out of a 43mm case. This little puppy in an FN would be cool, not to mention that the FN would be about a pound lighter, and I think with 50 years of improvements and modifications we would be very happy to go to war with it, I know I would. Heck, a with modern powder and a VLD bullet this round would give the 6.5 Grendel a run for it's money I bet. Instead we have to have these silly arguments about a round and weapon designed with faulty data and rammed down the throats of the military by it's political leadership.