Because it was retarded so it didn't happen, it was just a filler movie to introduce a character.
We see goku searching his ki in space and he goes to damaged planets that are still intact, he only blows up one planet that was more like a moon cause it was orbiting the planet in wich Paragus built the castle.
Its ridiculous to suggest any dragon ball character could one shot a galaxy, it's awful.
Its just bullshit like superman sneezing a galaxy, fucking retarded.
And you are retarded for eating it up.
Super perfect Cell was only solar level and he was stronger than Broly