Replacement 68 340 four speed cams

Does anyone know why the 68' manual 340 cam only came in that year and not any other in the 4-speeds. I did install a 68' 340 manual cam in my 69' Dart when I rebuilt it in 87'. It runs great as my other 340 (70') with an automatic cam. I think, possible being a non - slip clutch (manual) it could run a little higher RPM engaged.