Why were the Dambusters able to repeatedly circle 3 different dams without any fighters appearing?

I have read quite a bit about the Dambusters attack on the German dams (surely the most extraordinary, and certainly the most famous, bombing attack ever ? ) and one question keeps coming up in my brain.
How was it that all these Lancasters were able to repeatedly circle three different dams with no interference from the Luftwaffe ?
According to "The Complete Dambusters" Joe McCarthy famously made about ten attempts to get the angle of his run into the Sorpe Dam correct before "Johnny" Johnson released the weapon and the Sorpe was the third dam to be attacked. Even more surprisingly Dave Shannon "spent thirty minutes or more circling over the woods beyond the [Mohne] dam waiting for his turn to attack", before it was realised the dam had been breached and so he then set off for the Eder and made "several attempts" before releasing his mine.

 

Limited night fighter resources devoted to protecting cities/factories. Brits had a tigher OODA loop.

 

Colorized Dambusters documentary film (shot 15 min)

 

No way to test a non-falsifiable proposition. If it could be tested, you don't believe that Pearl Harbor and the A-bombs, maybe even Doolittle's Raid, would finish higher?