I remember reading an interview where Yoshi-P said that they didn't want to add viera because they were to similar to Female Roegadyn and Elezen which are very unpopular with players.

It was also stated that this is the reason why Au Ra was created smaller and less beastlike due to the unpopularity of tall races.

This reasoning is very similar to F2P MMOS like Tera where they make special jobs/gear only for popular Races(Elin), its because they want to market for the majority of whats popular rather than the minority, for the fear no one would use them.