The reasons for the successful launch of first iPhone introduced in late 2007 are below:
The App store
One of the main things that developers wanted was a shot at making awesome applications for the iPhone when it was first launched. Apple eventually let 3rd party developers in and that is when the iPhone gained more momentum. This further expanded the applications available such as software for creating and viewing office documents, games, applications for viewing updates from websites and programs for chatting online. Furthermore, the iPhone makes the installation of these applications in a way that any user could understand and it does not require any technical knowledge. The App Store keeps consumers engaged with Apple's mobile products.
The fees charged by these applications in general are not very high thereby attracting many users to download them to have the network effect. As a result, it helps both the developers and Apple to attract more customers and increase their revenue.
Lock in effect
The iPhone was an instant hit because many people were 'locked into' the iTunes ecosystem with the iPod already.
The iPhone is also an iPod. Using iTunes, you can synchronize audio and video content from your computer with the device and enjoy media on the go. The iPod is the world's most popular portable media player; many consumers are comfortable with the iPod's interface and already use iTunes to manage their media libraries. That makes an iPhone a natural purchase for a satisfied iPod owner who needs a new mobile phone.