Another marketing strategy is to sell certain varieties of Kit Kat only in some provinces. This way when people travel around Japan, they notice the specific varieties of the regions and even buy them as a present-souvenir for friends back home. This is the map of the different kinds of Kit Kat around Japan:
The most desired Kit Kat version in Japan (according to this poll) is the “yubari melon” flavor one.
The Yubari Melon (夕張メロン) Kit Kat is at the same time on of the most difficult varieties to get.
This one is a purple sweet potato (紅いも) variety that can only be found in Okinawa and Kyushu.