There have been some retcons regarding this though...
-First Death Star was severely sabotaged by Galen Marek in The Force Unleashed
-They couldn't get the laser to work properly (so a prototype only featuring the laser only was build before work went on)
But still, it doesn't explain why it took 17 years.
I believe infrastructure at the time of the second Death Star was much better, since ALL worlds had pledged loyalty to the Emperor, opposed to the time when the first one was build, when the senate was still operational and transporting materials from and through other star systems might have been difficult.
"I think the Devil ****s Dutchmen."