Yeah, story based. As your query is written, the answers will largely focus on the choices the Khan makes: why hasn't he sought to obliterate them? In other words, the question is leading towards answers that speak to the choices and actions of individual characters within a narrative framework.
An example answer would be: the Khan has not chosen to obliterate or drive the Orcs away because they're kind of dumb and make good buffers against potential enemies; plus, if you're strong willed, they do what you tell them to do.
Another example answer would be: the Khan has not chosen to obliterate or drive the Orcs away because his favourite wife is an Orc and she's the real power behind the throne.
So ya: you're going to need to edit your query to change its focus from actions of characters to fundamentals of society & culture.
I think it's kind of obvious, but I'd start by deleting the bit where you ask "why hasn't the Khan slaughtered or driven the Orcs out?" That's a dead ringer for story based closure right there!