Serious question: why would a non-poorly trained army select a more maintenance-heavy weapon? Would it have other benefits?
AR-15 base weapons (M16, M4) are much more accurate. So there's an advantage at distance. Also it uses smaller, faster rounds that have less stopping power, but are easier to carry. So there's a greater chance of running out of ammo with an AK-47.
Tighter tolerances can produce higher rates of fire and improved accuracy at the expense of reliability.