Influence!!!

A Nature was a condensation of power—that could be unleashed through Prana—derived from a living being's most natural of states. This stayed true for all Pranic Beasts.

Therefore, their Primary Nature was a direct representation of their state, perfectly attuned to their mental, physical, and spiritual needs, wants, and capabilities.

As a result, the Primary Nature of every single Pranic Beast part of a race is the same. This was the natural state present in Sumatra Continent. The sole exception were humans who went against the natural order of things.

The Human Race was the sole race with the freedom to obtain any Primary Nature as they pleased, which was a result of the mineral—of their choice—they built their Human Avatars with.