Because the planet's have giant shields over them which can only be disabled by destroying a power station on a smaller nearby planet?
Hmm, wrong movie i think....
But seriously, they do mention in the movie that there is obviously more than one planet, and if you think the planets are about the size of earth each, maybe a little smaller, it would take hundreds of nukes to wipe out all of earth, so theyd need thousands of nukes to wipe out loads of planets?!
Just a theory.