Gods you are so fucking retarded. Being a domestic group has nothing to do with being labeled terrorist or not. As to not designating those white supremacist groups as domestic terrorists, I think it might have more to do with the definition:
https://www.fbi.gov/investigate/terrorism
While those groups are despicable and despised by most people, they have not, as of yet, committed violent criminal acts to further their political goals. They talk a lot, they haven't DONE much of anything. ANTIFA, BLM and others, wellllllllll, they are much more active in the "committing" area so yeah, terrorists.