1. For broad consumer uptake, authentication methods better than a password, or indeed classical 2-factor authentication, needs to be "fire-and-forget". That is

1. For broad consumer uptake, authentication methods better than a password, or indeed classical 2-factor authentication, needs to be "fire-and-forget". That is, users set it up at most once and then it works almost transparently in most situations. This necessitates broad (read, universal) device-to-device connection. 2. There's no universal method for connecting a mobile phone to another device. 3. Besides, what if you require authentication without a password, on a mobile? 4. The closest to a universal method for interdevice connection is USB. Most importantly, a browsing-capable device almost certainly has a USB interface. Hence, efforts like Yubikey.