[an-unknown]

Simple: a real 3.5" floppy disk drive has moving parts and various things that age and eventually break. For example I have an old device with a broken floppy disk drive which can't even read a real floppy anymore. With the metal floppy "emulator disk" you mentioned, the FDD itself still has to be fully functional in order to read this "emulator disk".

A floppy emulator board which reads SD/CF cards or USB sticks doesn't have that problem at all since it's purely solid state electronics and directly connected to the electronic interface of the FDD instead of the real FDD, and usually you can put thousands of floppy disk images onto such a memory card/stick and select which disk image is to be put into the emulated floppy disk drive ⇒ there is simply no need for the "emulator disk" technology you mentioned anymore.