The reason they gave us one gender of each race, is because they're not sure we'll ever get any more races, and wanted to squeeze into two more somehow.
Adding new races increases their workload exponentially. Suddenly you have to create 16 variations of every single one of thousands of old and new peices of equipment instead of 14.

If they ever add in any new race options, it's only going to be male Viera and female Hrothgar, or nothing at all.