The Clone Army was just created and the plans for the deathstar are still in alpha testing (
With the empire being in full power (around Episode 4-6) it took approx. 5 years to build the (not completed!) second deathstar.
With the senate in his back and the Jedi still posing a threat, it sure takes at least 15 years to build a spacestation of that size!
And don't forget that he had all the civil workers on the second Deathstar! "Oh, those damned rebels! I am in the trade union!"