This has happened because of the population explosion. Reincarnation on new worlds does not happen because of every intelligent life that harbors the possibility of this. We call these aborted souls. This is not any decision on the part of an administrative force, it just simply happens. Nothing sparked a soul.
Nothing sparks a soul in a parakeet or a dog.
If there is an indolent life, or one without challenge, frequently the soul dissipates upon death rather than remaining to reincarnate and grow and increase in mass.