I would surmise that this is because Apple relies upon the PIN as a failsafe in the event that the Touch ID sensor can't / won't read your fingerprint. Let's say you only have your right thumbprint scanned and you injure it to the extent that it's no longer recognizable to the Touch ID sensor—what then?
Two-factor authentication would need to rely upon a much more reliable criteria than Touch ID.
Two-factor authentication would need to rely upon a much more reliable criteria than Touch ID.