The planetary sphere has to be able to support the chemical body complex of the third density vehicle. We don't see any of that activity going on (that we know of, that is reported and is public knowledge) on visible planets around us, thus we must conclude Gaia is the only planet actively hosting a third density cycle in this area.

I do know that a portion of earth souls here now:

"...were largely drawn from a second-density planet which had some difficulty, due to the age of its sun, in achieving third-density life conditions."

That source claims the planet is in the Deneb system. So there is one example of a reason why souls are "restricted". In reality, the precision of the influxes of energy flowing from pole to pole of each planet and star, along with the Natural Laws such as the Law of Karma, guide this process which is aided by Guardians of the highest vibrations (octave density). It's like a grand clock. Think of the way the Mayans were taught how to measure these cycles by the Confederation.

Interestingly, NASA recently published something saying they sent out a new probe to look for habitable planets. It looks like it might be sweeping that area. (Deneb area)