Noobish question

[Dodanodo]

this is probably an incredible NOOB-question, but I don't know that much about Japan, so here it comes. brace yourself:
why is the big Island north of Japans main Island, in present day a part of Japan, not represented in the Shogun period? was it not a part of Japan? and even if it wasn't, why shouldn't you be able to conquer it?

[Dodanodo]

ok thank you, that clarifies things. BTW +rep.