In addition to the other answers, the Constitution also notes that Lead Links can specially appoint additional persons. Specifically:
2.3.4 Special Appointments of Core Members
The Lead Link of a Circle may specially appoint additional persons to serve as Core Circle Members of a Circle, beyond those required by this Constitution, and may further remove these special appointments at any time.
We've been using this to appoint a non-Circle Member as Facilitator, especially right after creating a new Circle. This to ensure meetings are lead by an experienced Facilitator even if none of the "real" Circle Members have (yet) gained these skills.
Back to your question "Why is it best that facilitators are members of the circles they are operating in?" - I was not aware of Holacracy explicitly mentioning this as being "best" (as in preferred over a non-member). When a Circle has been under way for a while, it does feel like a natural thing to elect a core member as Facilitator, but that is not necessarily entirely the same thing.