Introducing male Viera wouldn't be that hard. SE writes the lore. They could easily come up with reasons for us to encounter male viera. And in terms of PC male viera, that's up to the players to determine for their own headcanon backstory.

Lore is not the reason we don't have male viera, and lore is not the reason SE isn't likely to add them in the future. It's really jus ta matter of what resources the dev team has.