From a gameplay perspective, there isn't much sense in a full N-body simulation. Players only care about their spacecraft, which is a restricted three-body problem (small spacecraft, massive gravitational bodies) outside of special cases like gravity tractors. And the simplified two-body SOI worked pretty well in practice.
In any case, the bigger gameplay problem tends to be the vehicle physics simulation, where you have hundreds+ parts interacting and not in the most stable fashion especially above 2x time warp. That's the part that seemed amazing when KSP first came out, moreso than the orbital simulation (although the solar system exploration clinched the overall experience).
