Passport is not just aimed at solving the problem of single sign-on. Passport is designed to support most existing browsers (as opposed to custom software). It allows users to change machines (a problem when you have to take a ZKP key with you) and it stores all of your personal information in one place - making it easier for you to update your information and guarding against attacks on various sites.
Granted passport does very little of this well or securely, but a ZKP based solution would require more infrastructure.
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Makes you wonder what Steve Jobs' bill looks like...
Granted passport does very little of this well or securely, but a ZKP based solution would require more infrastructure.