They have almost zero space presence, they only attack the ships from the ground. The only thing they can do with space is sending eggs away. (And those asteroids weren’t knocked out of orbit, they were sent though hyperspace.)
Besides, the movie makes it pretty clear they just discovered they are in a war a few weeks prior.
(And now I’m wondering how the fuck do I remember that well a movie I’ve seen once, a long time ago… Is it actually good and I didn’t notice at the time?)