It's not that they don't want more races, but that rigging gear for another set of models is intensive, and going back through all the existing gear that's amassed over the years and remodeling every piece for the new race is just... I'm amazed they're still tackling that with male Viera and female hrothgar TBH.
It's not that they don't want them, but ho boy is it a prohibitive amount of work...