As far as I can tell, the reason Gallia has been less than popular is because:
6. Their tech is incompatible with the rest of Sirius (they don't share).
5. They are inconveniently located at the farthest upper corner of the map where few players bother to go.
4. Their NPCs are uber buff and a continual pain in the rear.
3. France just isn't as intriguing as Germany, Japan, or Britain.
2. France has never been known for it's military prowess outside of the Napoleonic era, yet they are single handedly bulldozing the entirety of Sirius for... revenge? Seems like they have self-esteem issues.
1. There was little to no desire to bring in another house, regardless of who that house would have been. Players wanted the existing house lore to expand and improve based on player interaction. That didn't happen.
