The article had a link to an AK-74, and I thought is that a typo for an AK-47? Apparently Russia moved on to the next generation in 1974 and the rest of the world remained with AK-47s?
During WW2 and Korea, militaries generally favored larger, heavier bullets. The thinking was power is good. Over time, people realized that carrying weight is bad, and volume of fire is usually better than the power of a single cartridge – so the US moved to the M-16 (M4, AR-15) and the Soviets to the AK-74.
The sticking point is you have to have the money for BOTH:
1. Mass caliber conversion (rifle replacement, new ammo, new tooling...), AND
2. Purchase of additional, larger caliber weapons to replace the capability you lost when switching most of your forces from the intermediate cartridge to the newer, smaller bullet.
So, poorer countries tended to stick with the AK-47, and wealthier Soviet-bloc countries went to the AK-74...but there aren't too many wealthy communist countries, so the AK-47 remained the standard.