Do you think Gallia would be so stupid to blow the planet? Their only "Gas station" - a planet? They need a staging ground and the planet would be excellent place for that. Ofcourse its not so easy to conquer it all - maybe they will get some of its landscape but whole of it? Definitely nope.
GRN from what I know belive that Bretonia is "conquered" cuz only one system remains to be head chompted - New London. The real deal is that currently, after the gates being destroyed - GRN are unable to send their armada and capture the capital. So they turned their gaze towards Liberty, which are coming for them. The liberty forces, considering that fact they are no longer fighting Rheinland, are the strongest and most advanced in Sirius - and currently the strongest foe. GRN belive that if they conquer Liberty, they can return and finish Bretonia with ease.