[citation][nom]krowbar[/nom]Who would want an old iphone anyway? There are much better phones, especially if you have to get in a 2 year contract, you can get better ones for free or cheaper! Android and WP7/8 are better choices IMHO.[/citation]
Because people want iOS Apps, which are more plentiful and better than any other platform.
Use the songs, videos downloaded from iTunes easily onto your iPhone 4S, without any fuss (there's millions of people like this).
Easily move your messages, photos, contacts, etc. from an iPhone 3G or 3GS with a few simple clicks onto a 4S.
Know that a 4S gets updates right away, and not wait like the Galaxy S2 forever just to get JB.
Know you can always pop into an Apple store to get help. Not phone Samsung who then just gives you a phone number to some random place in your city who charges 100-200% more for fixing your phone. Or you could spend 3-4 hours phoning all the shops in your city to find the best place, or drop by an Apple store to get full customer service that is unmatched in the mobile sector.
There's more reasons too that are quite self evident that makes the iPhone the most popular smartphone in the world.