Also remember 12,000 years ago the Flood of Darkness tipped the balance of the remaining shards out of balance, and the first was tipped closest to light from the onset several millenia even before the flood. The elemental polarity being naturally shifted more towards light might have been an environmental factor that led to Pixies coming into existence.

It's also possible that Pixies did exist on the Source, but maybe were entirely wiped out in any of the 7 calamities that hit the world, whereas the First never had a calamity besides the Flood of Light.