Perhaps the same reason my British ex-pat fiance won't file for his American citizenship despite having been US resident alien for 7 years and having zero intent to ever go back to England other than as a visitor: the US taxes American citizens living abroad. There are tax treaties in place with many countries where, for all intents and purposes, your foreign tax paid to your country of residence may be deducted from what you "owe" to the US government but it's a mess of forms and complicated filings. My mister refuses to go for the US citizenship just on principles. If we were to move to one of the Commonwealth countries in the future, I'd likely renounce my US citizenship once I were able to gain citizenship in my new home country; however, interestingly enough, it's illegal to give up your US citizenship for the sole purpose of not having to pay US federal tax any more. So I'd have to come up with a different reason

Granted I've not looked at the laws in a few years--- i decided long ago not to press the citizenship issue--- so things may have changed a bit.