1. Im with AT&T and Im in an area with no reception issues
2. The only phone other then a prepaid one Ive ever owned is an iPhone
3. Im comfortable with the operating system and its design
4. You cannot fault the build quality of an iPhone, the 4th gen I have feels very stout and solid
5. The display is top notch, best Ive ever seen and does not strain my eyes
Those are a couple reasons I go with the iPhone. It works for me and I am satisfied with it. Ive never given any other "smartphone" a chance because I havent felt like I wanted to and I like what I have.
Edit: If anyone believes I am an Apple fanboy of sorts you probably would not be far off target. I dont mind paying a premium for their products because I have always been very satisfied with them.