[Akuma]

How do you know that they hadn't been building the second Death Star for years? It could have been a secret that only the highest ranked people would know about.


Also there was no real need for the death star after all the Jedi were dead. No one was a real threat at the moment. So they slowly build it as the rebel threat grows.

In the beginner of RotJ, the guy in charge of building the second death star says that his men are working as fast as possible and that the Emperor asks the impossible.