Quote Originally Posted by Aelsioln View Post
Don't quote this anywhere because I can't find the source atm but I swear I read somewhere that when they were planning on adding the last race in Shadowbringers they ran into a problem. The team really wanted to add in a more beastly race since they felt such an option was lacking at the time but they knew that players really wanted Viera. Hence the compromise we got of one gender of each race to try to please everyone which didn't work since people immediately protested the idea of only getting half of each race.
It's the Letter from the Producer LXII.

Some key paragraphs:

In a Fan Festival keynote speech, I stated that this will likely be the last time we add new playable races to FFXIV. To put it simply, a tremendous amount of time and resources are required to develop one.
Since this may be the last time we add new races, we spent a long time deliberating which race would be the best addition. Fans worldwide have shown an overwhelming desire for FFXII’s Viera [...] and so we decided to try and satisfy their request in this expansion. The desire to please our fans is the primary reason behind this decision.
[On Hrothgar] After seeing the positive reception of the Lupin characters we introduced in Stormblood, we concluded that, for the long-term health of FFXIV, it was important to implement a greater variety of character types beyond the traditionally beautiful in a future expansion. In truth, we had considered adding more beastly characters long before, and since this was possibly our last chance to do so, we decided that we should—much like we did with the Viera—in the interests of expanding the lore and the choices available to players.
[We] felt that it was in the best interests of our players to try and satisfy both demands. We understood that some players would be upset if they were unable to use their preferred gender, but even so, after much internal discussion, we decided this approach was the best way to improve the overall game experience.

We could have decided to implement only male and female Viera, or only male and female Hrothgar. However, we instead went with an approach intended to expand the possibilities in multiple ways.