Why do Western aircraft manufacturers use FPS system while Russian aircraft manufacturers use MKS system for flight instruments?
The short version is that "Western" countries (nearly all countries, in fact) use knots and such because that's what we started with and, once that became entrenched, the cost of retraining every pilot and reequipping every plane--and the inevitable confusion and crashes during that transition period--greatly exceeds the benefits.

The USSR, PRC and a few others were able to use metric units because they didn't need to care about compatibility with the rest of the world--or perhaps saw the lack of compatibility as a good thing, helping to keep them isolated.