Because it worked so well the first time, right? So why not build an identical one, with the same apparent design flaw that leaves it vulnerable to instant destruction by a well-aimed torpedo? The second Death Star is proof that, either the Empire was running out of fresh ideas, or Lucas was.
If anything, the second Death Star was a worse idea than the first, since – as Star Wars fan Kevin Smith’s characters would point out 11 years later in Clerks – it presented the rebels with an ethical dilemma. After all, the first Death Star was finished when the rebels destroyed it, so presumably only imperial troops were aboard. But the second Death Star was still under construction when rebels blew it up in Jedi, so there must have been thousands of slave laborers or independent contractors aboard, innocents who didn’t deserve to die.
Not that any of this seemed to have crossed the minds of Leia or Han or Luke or Admiral Akbar – or Lucas himself, for that matter. Even with all that, the second Death Star was still a better existential threat than, um, whatever the Big Bads were in the prequels to come.