It's trivial to remember a very long password.
It's obnoxious to have to enter it on an iPhone keyboard with one hand while trying to drive or whatever on every login to anything. Particularly bad since I'm on a platform with a secure element, protected by tamper-resistant hardware, and relatively decent OS provided APIs to handle this exact problem.
There are good rumors that iOS 7/iPhone 5++ is going to have at least a fingerprint sensor, too, which would then automatically give biometric protection to the keybag as well.