iOS asks the user for their iTunes password for many reasons, the most common ones are recently installed iOS operating system updates, or iOS apps that are stuck during installation.
As a result, users are trained to just enter their Apple ID password whenever iOS prompts you to do so. However, those popups are not only shown on the lock screen, and the home screen, but also inside random apps, e.g. when they want to access iCloud, GameCenter or In-App-Purchases.
This could easily be abused by any app, just by showing an UIAlertController, that looks exactly like the system dialog.
Even users who know a lot about technology have a hard time detecting that those alerts are phishing attacks.
These dialog boxes pop up all the time and users who aren’t tech savvy can, will and do just type in their passwords without thinking about why they are doing it.