The iPhone 4, 3GS and 4S are the most popular smartphones in that order. Why?
1) Simplicity. A lot of people want a smartphone with as small of a learning curve as possible - in other words, one that "just works."Apple prides itself in delivering that phone. For example, I can switch between apps simply by double-tapping the home button, swiping left or right if necessary, then tapping the app. I didn't have to tweak the phone so I could do that.
2) Siri (iPhone 4S) is currently the only "personal assistant" I am aware of that is available on a cellphone.
3) Some people have or had an iPad or an iPod touch. So using an iPhone is even easier than it would be if they didn't have an iDevice. Plus, the apps that work on an iPod touch also work on an iPhone.
That's my list of reasons.