Who's to say he didn't? They left a lot to be uncovered. If that's your only defense then
And we're talking about tdk here fella
I know we're talking about TDK but it's within the series. The Lazarus Pit was shown as a prison so to speak so you're wrong on that one. Nolan said plenty of times that he wasn't going to use the over-the-top mystical approach regarding Ra's al Ghul's background. This same tactic went towards other characters and their traits that wouldn't have made sense in Nolan's reimagining of the story.
Once again, TDK was a good film but not a great comic book film...hence why it's overrated for me
Something along the lines of "Demons don't come from the hell beneath us. They come from the sky"