The only write once media I'm aware of that is in significant use are WORM tapes. They don't offer significant advantages over regular tapes, but for compliance reasons it can be useful to just make it impossible to modify the backups.
Because the older Flash aren't as stable when miniaturized as you'd expect. Current flash is a direct descendant of these, they only are more stable because the cells are much chunkier and thus with lower leakage.
That being said, it's not clear how innovation has occurred in this direction in the storage space.