Endor was a trap, why would they move the death star? It was meant to be the lure. No Death Star, no trap. No trap and the war continues. The Emperor never listened to Han Solo's advice and got cocky. He should have waited until the station was more complete.
Of course thinking about it that's the entire point of the opening scene. The Death Star II was behind schedule when Vader arrives so it's not as much the emperor being cocky but not being informed as to the actual status of the station. At that point the rebels would already have gotten the plans and making preparations for the attack. So if ANYONE is at real fault it's Grand Moff JerJerod or whatever the guy in charge of the DSII's name is.
Edited by Forresto