Because sci-fi writers have a hard time thinking and writing 3 dimensionally, they have a very bad tendency to think linearly in a 2D fashion. They always seem to forget that in space you can also go up and down in addition to left and right and in the case of a planet, there's more than just one face to it and to blockade a planet you need to pretty much completely surround it because a planet is not like a harbor or port, or even a space station. All they really had to do to fix it was to toss in a simple line saying that they had the planet completely surrounded, even if we only saw the fleet in "front" of the planet.