I'm only guessing here. I'd have to say that since there weren't a large number of moves back in Gen 1, GameFreak might have chosen to bolster Pinsir's level-up movepool with a move that somewhat made sense. However, since Pinsir's horns don't really make a slashing motion but more of a crushing motion, GameFreak may have decided in Gen 2 to replace Slash with other moves that made a bit more sense.