I started seeing them around this tiime. http://en.wikipedia.org/wiki/1979_energy_crisis People were stealing gas.
Caps are keyed to prevent theft and tampering. It would suck to find out someone decided to dump sugar in your tank.The keyless caps that are out there are intended for track bike use. If you're bike has no ignition lock, it doesn't make sense to still have a key for a gas cap. Locks not only add unnecessary weight, they add complexity. I remember being at a track day where a guy ruined his day because he forgot the key to his gas cap. Dohhh....