There are three reasons:
1. Lot of people don't want to change their carrier.
2.Few people simply think that iPhone is lacking some feature like video recording, MMS, cut, copy,past, Live TV(which is not true iPhone does have live TV, live photo streaming,Cut copy paste iPhone just lack video recorder)
2. It's many people's tendency to make an opinion against something that is so hyped or successful
I was against Apple till I won 80 iPod video in a competition and after that I became a die hard fan of Apple. After that I sold my Toshiba laptop, Sony desktop and Laptop and bought MacBook Pro 24" iMac and iPhone.
iPhone is the most advance phone on the planet you can figure it out by the fact that apart from many free app there are many app for iPhone that is selling for over $400. That shows big corporate house es are using these apps on their iPhones. One of the iPhone's app is selling for as high as $900.
I am myself a SAP consultant and currently using one app called SAP Business One it costed me $449 but its worth every penny it helps me in job in a way that no computer can do it.
No wonder iPhone became the most favorite phone among business user and most loving phone among youths. On iPhone you can make 3D stop motion animation movies which is simply awesome.
iPhone is an entertainment center I don't feel like leaving it I keep on doing something on it.
@Andrew
Iphone also can download application right from the phone and it has voice dialing. Camera is decent. And the quality of camera doesnt not depend only on MP.
I have 6MP SLR and there are many point and shoot camera available in the market that have 9 MP. Do you think those can beat SLR ? no way.
iPHone also has street map and google image. Apart from youtube,metacafe,google video,vSnax video and many more iPhone also has Joos where you can watch full length movies for free. iPhone has ability to do VoIP.
iPhone has thousands of features that your phone doesn't have. Have you heard of iTunes U. Universities like Harvard, Stanford have made tutorials.
The awesome video podcast. Games like Brothers in Arms.
You can use iPhone even like measurement tape. No wonder it has become the highest selling phone in US and the most favorite phone among business users.