Simple answer: There was a war.
After the WWII, many patents owned by German companies were taken by the allies as war reparations. In an interesting turn, the US decided to play God again and gave significant funding and technology to Japanese companies (even though they were the ones who originally attacked the US). That's why a Nikon I had a Leica shutter but a basically Carl Zeiss lens mount and body shape.
The biggest companies in Germany managed to bounce back slightly but almost all collapsed before by the 1970s, while the smaller ones were wiped off the face of the Earth (usually by Bomber Command or the 8th Air Force). Many firms had their plants (some their entire operations) in cities like Dresden or Leipzig, which pretty much ceased to exist as something other than a burning cinder (again, thank the Mighty 8th). Add to that the country being split in half by bickering Allies and the combined loss of, oh, 10 percent of every man, woman and child in the country, and you start to see how bouncing back became difficult
Yes, there was technology transfer to Japan after WWII, but what really spelled the beginning of the end was high-quality, original, affordable designs that came a decade or more afterwards. The most influential being the SLR. A German idea, originally, but complex, expensive, clunky, awkward cameras, until Pentax and Nikon took the idea and ran with it.
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