The Death Star is as big as a small moon. You probably want to jump in and out where you’re less likely to run into something. It might even tax the systems needed to power the superlaser. Even the second Death Star can’t pop off several shots at once.
There’s also the psychological terror aspect of seeing that indestructable sucker slowly coming at you. Everyone on Alderaan had plenty of time to contemplate their doom even if they didn’t know what the DS was going to do.