Same as any other mala prohibita law. At some point, switchblades came into favor with criminals and thugs. Scared, yellow bellied, whining sheep cried to their politicians, who saw an opportunity to win political points by pretending to be tough on crime, when in fact they were not.
Never mind that using any knife on another person while not in the act of self-defense is a crime, let's ban the tool! Tards.
This.