WBH, many other countries do not have so many immigrants like the USA. If only 1% of your citizen population are those who naturalised, you would not want to allocate a special field, right? All passports looks quite similar in order to speed up the immigration clearance in any country because border agents already know where to look for a particular date. An extra date in the passport would probably confuse them, and also every border agent in every country you visit will know that you are not a native-born citizen of your country. Would anyone agree to be distinguished in this way just to remind themselves when they became US citizens? Plus, all native born US citizens would have two fields that show their DOB - a complete nonsense.