Why is a Canadian passport only a prima facie proof of citizenship in?
A US passport is prima facie proof of US citizenship just as it is in Canada. I have no idea why you think a US passport is treated any differently. Passports are often primary evidence of citizenship in a country. We dont go around carrying birth certificates or naturalization certificates to provide such proof unless we have to. The only time Ive need my naturalization certificate to prove citizenship was to update my Social Security records. Even then I could have used my US passport if I wanted to. Either document is acceptable but the naturalization certificate is just easier to use in that case.