You have a clear view of what you want for Jersey, that is known and could be added if it was deemed necessary.It is a proof systemic from the reasoning. There are places less historically, economically, militarily less important than both Jersey and Japan that have received more attention from Paradox. You are meant to supply a criterion by which they decide to spend time adding or detailing places that isn't my asserted one of "because they feel like it (or don't)".
Japan doesn't have an obvious solution. Should there be one country with revolt events? Should they add more tags and more detailed interaction between Daimyos and the Shogun? There is no clear answer and doing the wrong thing could be worse than doing nothing.
I may be wrong, but when they redesigned Japan in EU3 Divine Wind, the response was "its a bit boring" or something not terribly positive. Maybe they want to avoid that result?
What I'm saying is, Jersey isn't in the game because its fairly insignificant, so they haven't added it. Japan hasn't been changed because they haven't figured out how to change it.