Why are there pixies, etc. on the First but not the Source?

[Brightamethyst]

I have to wonder if this is actually a decision made out-of-universe to allow for the newly-designed race to appear in the expansion, rather than actual lore.

It's almost certainly primarily a gameplay decision, but there are any number of legitimate lore reasons a move could have happened if they ever need to explain it. For all we know the Viera are native to Eorzea but moved out in the wake of one of the calamities.