Except for the WoL, there is not a single Viera, Au Ra or Hrothgar within the Scions. No new race made it as a new member of the Scions. Basically, Elezen are leading the Scions with the most members in it.
The first part makes sense, because the Scions haven't really expanded at all since the Crystal Braves era. There hasn't been anyone of any race joining since Viera, Au Ra, and Hrothgar came out.

Elezen may be "leading" the Scions in numbers, but they're not a majority. 4 Elezen vs. 3 Hyur and 3 Mi'qote isn't exactly an overabundance.