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?

 

@Riter makes a couple of good points. The German night fighter system was geared to engaging bombers at medium to high cruising altitudes en route to their targets. Radar needs a clear "line of sight' to its target. Detection "under the radar" is difficult and shorter-ranged due to terrain and the curvature of the earth. It was also more difficult, especially at that time, for airborne radar to pick up targets below the aircraft or to distinguish them from the terrain.

In the Luftwaffe system, each ground station controlled one night fighter and would direct it to a bomber once detected. The fighter also had radar, but it was short-ranged and only scanned forward, so interception depended on the station coaching into position where it could pick up the target itself and attack. This may sound clumsy, but they destroyed quite a few bombers, and it was not uncommon for fighters to get multiple kills in one flight.

It's tempting to think that someone at the dam could just ring up the Luftwaffe and have fighters leap into the air but realistically it would take time for people to assess the situation and decide what to do about it. No doubt there were lots of reports night after night of Allied aircraft doing various things. Do we know what other raids or activities were going on that night? It would make sense to schedule Chastise when the Germans had other things to worry about.

Most of the available night fighters and crews probably already had missions assigned. Actually, day fighters might be usable, if aircraft and pilots were sitting around ready to go in the middle of the night. As noted earlier, radar interception would not be feasible; it would be more like "Fly to the Sorpe dam and look for British bombers" (likely reply: "What's the Sorpe dam?")