If I remember correctly, there's some ambiguity whether the slow zone was created by some power or naturally occurring (or a bit of both) although a bit of both is perhaps the natural answer.
I remember some insinuations to that effect, but from an unreliable narrator given Vinge's penchant for strictly in-character exposition, and what would child races know of progenitor's motives?
As a universe arrangement scheme (both narratively and literally), it is a clean solution to providing a new race incubator and preventing "Why don't the First Ones rule everyone?" endgames.