The balisong, also erroneously called the butterﬂy knife, has been around since approximately 1200 BC where it played a part in the Filipino martial art form, Escrima.
Although some commercially available ones have been around for many years, they never really caught on until the 1970s.
I know one knife-maker in NZ who makes balisongs exclusively. The reason for the balisong’s popularity: four pins, two handles, one blade. No springs, screws, rivets.