This is just my personal observation with no data but AFAICT Kit Kat is *mostly" for tourists here in Japan. If there's an obsession it seems to me it's tourists buying them. I could be wrong but I've never personally seen Japanese people buying them (of course they must, just saying that I haven't noticed). I just walked over to 3 convenience stores that have a large candy selections. The 7/11 had no Kit Kats out of…
There was also the marketing ploy around 15? years ago that they did a play on Kit Kat to be きっとかつ/きっと勝つ/kitto katsu for students that wanted to feel like they would surely win a game or pass a test.