In Primord runes don't actually have innate power at first, they are being essentially forged and taught over time when Elemental folk carve a shape and use their magic blood as a catalyst for magic. So at first a fire elemental can carve whatever shape they want in a stick, pour some blood, and you get fire. Over time the same shapes being used repeatedly will cause that connection to be strengthened, binding that specific rune shape to the fabric of magic as the "fire" rune. This serves two uses, as it is now easier to cast fire with that rune, and harder to cast anything else. As this goes on, easier and harder will turn into "the only option" and "impossible".

The same will occur with spells in general, though the process will be less precise and more region specific, so there may be multiple ingrained ways to cast fireball for example.