[bombcar]

Because by the time the 3.5s came out, "floppy" had become a word meaning "removable" instead of "bendy" - the alternative being "hard disks" which were fixed storage (internal or external).

[dragonwriter]

No, the media of the 3½” was just as floppy as the media that was the source of the name for 5¼” and 8” floppies.

It was never about the outer casing for any of them.