Why aren't diplomats allowed to naturalize?
The reason is that naturalization is not just the bestowing of rights and privileges. It's not just an "award" for time lived in a foreign country, or an acknowledgement of one's parents' nationalities that you inherited.
It's a Social Contract that binds a person, going forward until they die or legally give up that nationality, to a sovereign nation's laws. These laws include both current laws and any future laws, that you cannot foresee or expect now, that are created by the legislature — and the responsibilities that those laws create.
You may not necessarily be able to necessarily escape those responsibilities by just leaving the country and living or working elsewhere.
Those responsibilities could, either now or in the unknown future, include overseas taxation, military conscription, or anything else.
When you become legally Japanese (or even legally American), you pledge to one or more legal oath(s) swearing to abide by the supreme law of the land (the Constitution) as well as all the other laws. Oaths are not empty promises. They mean something.
On the other hand, the whole point of diplomatic immunity is that a diplomat is not held to the laws of the land they are stationed at. Thus, the special protections that diplomats receive from the Vienna Convention are in direct conflict with the oath a naturalized person takes.
In a similar vein, diplomatic protection is usually extended not just to the individual, but usually also to the immediate family accompanying and living with the diplomat overseas and their overseas home and vehicle. For that reason, a child that is born to a diplomat in a jus soli country, such as America, does not automatically receive U.S. nationality like other people born on United States territory. This is reinforced by the citizenship clause in the U.S. constitution's 14th amendment: the only persons Congress intended to exclude from birthright citizenship under the 14th Amendment were children born to diplomats.
All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside.Canadian law, which is primarily jus soli like the United States, also does not allow for the children of diplomats to automatically acquire natural-born Canadian citizenship, as per §3(1) of their laws concerning acquiring Canadian citizenship by birth — the "diplomatic exception":
[You do not become a Canadian citizen if you] were born in Canada but were not a Canadian citizen at birth because when you were born, one of your parents was a foreign diplomat and neither of your parents was a permanent resident or Canadian citizen;Of course, a former diplomat is free to naturalize. And it is also possible to be a diplomat of one country and acquire the nationality of another country that one is not a diplomat of.
