They spent over half a year using what free time they had in between their normal work on the patches to consider ways to add another playable race. From this they concluded that we could probably squeeze in just one more at this time.
Adding a playable race does not end with just the initial resources needed to create the race (i.e. character modeling; preparations for variations in facial features, hairstyles and head shapes; animations that are unique to each race, etc.). Even following initial implementation, the tasks required with each patch update will multiply. For example, when we implemented the Viera, because their ears are very long, we needed to add a new review flow to check how Viera characters would appear on screen in all cutscenes. Of course, gear you can obtain as you play through the game will also need to be fitted to match the body shapes of the Viera and Hrothgar, and whenever we add emote animations, there will always be two additional sets to consider compared to what we had to do up until this point. This is something that we as a development team need to put effort into, but if these tasks multiply too much, it could cause critical damage to the interval between major updates.