Because leaving flaps down after landing used to (maybe still is) a ground signal for hijack.
Leaving flaps down, or partially down, after landing may be done because of snow, system failure or failing to do the checks!
Many years ago in the military, after landing at an American base, I neglected to raise my flaps after landing (cock-up) and found myself surrounded by flashing lights near the apron.
It didn't seem to matter to them that I was in a single seat aircraft!