Why did the concept of "write-protecting" not carry over from floppy disks to USB memsticks and especially external USB HDDs?
I'm not entirely sure that this question is on topic, but nonetheless, I suspect it has a simple answer.
Physical write protection mechanisms predate floppy disks. At my school holiday job in the 1980s, my desk had a drawer full of write rings for reels of 9 track tape. If the ring is in place, you can write to the tape.

There are certainly flash devices with physical write protection mechanisms. Many older USB drives have them. Memory Stick and SD both have a write protection lock. You can see it on the left-hand side of an SD card.

Micro SD cards don't have them because there's no way to effectively make them &quot;micro&quot;. And I suspect that's the main reason why they went out of vogue. Once it became unimportant for one removable medium, many of the rest followerd.
The history of storage is about maximising speed, and minimising size and price. Adding a write protection lock adds to both size and price when devices are very small.
In addition, moving parts wear out faster than devices with no moving parts. Unlike disks or tape, flash memory has no moving parts by default. Adding a write protection lock means adding a moving part, which effectively shortens its usable life.