One of the biggest advantages of the AK47 is that it could be manufactured en masse with relatively simple manufacturing equipment that was widely available and inexpensive in the middle of the 20th century. Consequently, it could be produced even in marginally developed countries as a matter of both technical ability and economics. Contrary to popular rumor, a number of countries produced AK47s with relatively tight tolerances so lack of precision machining was not intrinsic to its success.

By contrast, and many people forget this, when the AR15 family of rifles was first produced, only a handful of industrialized countries had the ability to fabricate the precision aluminum parts used in the rifle en masse. When the design was exported, the US often exported precision aluminum foundry and machining technology at the same time. That limited adoption as a practical matter.

All that said, the reputation of both the AK47 and AR15 suffer from relatively isolated instances of defective implementation. A well-made AK47 has perfectly serviceable accuracy, comparable to many European assault rifles. However, its operational ergonomics are atrocious. By the same token, the AR15 functions far better in filthy environments than popular mythology suggests; most infamous incidents of documented systemic failure were ultimately traced back to out-of-spec components. An AR15 will not take quite as much abuse as an AK47, but it makes up for it by being one of the most highly optimized assault rifles in the hands of a skilled operator due to the unusual operating design.

In short, both the AK47 and AR15 offered compelling economics because they were designed for and used by disjoint markets with very different priorities.