Nestle responded to our e-mail on the question.
So, as you can see, it's some legal issue with Hershey's here in the USA, though it went unenforced for years, until about a year ago. You can still import Japanese Kit Kats through different sites, but it's still sad not being able to just grab them in our own Little Tokyo.
"Nestlé has granted HERSHEYS the sole and exclusive right to use KITKAT in the USA. This means that Nestlé cannot sell products, even if manufactured in this case in Japan, to a distributor knowing that they intend to sell these products to customers in the USA.
I hope this is helpful – I know many people love the different and unique flavors of Kit Kats from Japan."-Edie Burge | Corporate Communications
Nestlé USA