This is a very interesting question. I want to know the answer too.
I guess the main reason is a lot of 大陸女傭 will not want to go back to 中國 once they get a chance work in HK, which will create a lot of legal problem. 印傭或賓妹 has more credit on that.
Also, I guess HK government don’t want 大陸人 become HK people 傭人 . This sounds bad in the public.
I think the main reason is generally Hong Kong pople have lots of relatives in China, the government does not want they use the reason of employing domestic helpers to help their relatives to stay in Hong Kong.