I thought this Macworld article covered the answer pretty well. One of many convincing points:
But while [Keychain is] broadly useful in OS X, as more developers have adopted it and there’s Keychain Access for direct lookups and retrieval, in iOS you have to drill down to Settings > Safari > Passwords to view, edit, or (swipe all the way to the bottom) add passwords. Further, you can’t invoke Keychain in Apple’s non-Web login dialogs, making it useless for common purposes. And while you can make up a password when you need one, it’s awkward to get to and can only be retrieved easily on a corresponding Web page.