Why was the iPhone such a successful product?

The iPhone did away with physical keyboards and instead introduced a multi-touch screen which didn't require a stylus. The iPhone software, iOS, was developed to take advantage of all hardware and design capabilities of the phone, including the touchscreen. Each application presented its customized input options on the touch screen. In addition, the iOS was flexible and expandable through the App Store, giving the iPhone  user a wide choice of third-party apps which were not known before. iPhone hardware quality was also a nice surprise to the users.