Because once you become a foreign national you will need a visa to live in your birth country though technically you have to renounce your US citizenship. Debito Arudo has written about it
here and the US doesn't like its nationals giving up their passports.
JAPAN'S REQUIREMENTS
I went down to the local Ministry of Justice (Houmu kyoku) and sat down for an hour with an official. At first, he talked at me as if I were a child, and about very private things. When he started interrogating me about my parents' marital status I interrupted: Hang on. This is immaterial--all I want are the bare bones of what it takes to qualify, not whether or not *I* personally qualify, for citizenship. He nodded, hitched up his politeness level, and gave me the beef:
TO QUALIFY FOR JAPANESE CITIZENSHIP, YOU MUST:
a) have lived continuously (hiki tsuzuki) at Japanese addresses for five years
b) be over twenty years of age "in terms of mental and legal capacity" (20 sai ijou de honkokuhou ni yotte nouryoku o yuusuru koto)
c) behave well (sokou ga zenryou de aru koto)--and they do check--my dictionary even has the word "sokou chousa" (personal conduct survey) in it
d) demonstrate the means to support your family
e) be willing to relinquish the citizenship of your native country once Japanese citizenship is granted
f) respect the Japanese Constitution (i.e. don't plot against or advocate destroying it, or associate or join a group or political party which does)
(extenuating circumstances for the above considered if the applicant is married or related to a Japanese)