To create a giant superweapon is stupid. Better to create several small weapons that could effectively do the same thing without so much energy. They would be cheaper to build and maintain, could be in several places at once, and thus cause more damage without exhausting as much finite resources. Fact of the matter is, the Emperor is not necessarily interested in what is smart or most judicious. He is interested in what is most terrifying and morale destroying. What did he care most about during the Battle of Endor? The generator? The Death Star? The Imperial Fleet? Crushing the Rebel Fleet? No, he cared most about winning his prize apprentice, Luke Skywalker. And in a pinch, destroying the Rebels on Endor would effectively further crush his morale and drive him closer to the Dark Side,