In Rogue One: A Star Wars Story, we see Galen Erso (Mads Mikkelsen) being recruited against his will to help the Empire figure out the Death Star in terms of its actual use.
The first one was destroyed in Star Wars: A New Hope, which was able to be accomplished thanks to the plans obtained by the Rebels featured in Star Wars: Rogue One.
Thanks to Rogue One: A Star Wars Story, as well as the Star Wars novel Catalyst, we now have a very good reason for this.
Rogue One: A Star Wars Story is finally here, and the vast majority of fans and critics agree that it is a very good Star Wars movie.