Because people in Kanto nearly ate Farfetch'd to extinction there is only one in RGBY. But there are many in Johto thanks to the preservation efforts mentioned in Crystal's Pokedex. This gives weight to some common assertions on board that wild mons in GSC that can only be found in the Safari Zone in RGBY, are more closely related to Johto than they are the Kanto region. Examples include Scyther and Pinsir in the National Park.