They mentioned that has something to do with the amount of slots (for lack of remembering the correct term) that can be used for creating a character. The slots are like Size, Bust Size, Face types etc. and do not include what goes into them, like the multiple hairstyles that they can keep adding to. This is why not every race can adjust their muscle tone, ear shape etc. because the slots are taken by something else, like tail length. To make the horns of Au Ra and markings of Miqo'te seperate from faces, they would need to add another slot or remove something else, which they can't/won't do.
Edit: The slots were referred to as Memory Allocation by Bayohne
