They were designed with slightly different priorities. The AK is a battle rifle, plain and simple. It’s designed to reliably go bang and uses a beefier round than the AR. It is more dependable and reliable than the AR platform, but is also less customizable and versatile. The AR was designed with modularity and customization as a priority. The idea was to create a single platform that could be built on in different ways for different roles on the battlefield. An AR-15 can be chambered in a variety of different calibers, while an AK cannot. AR-15s are designed to have largely interchangeable components, while the AK was not.