Because unlike Android devices where someone can create a virus or malware with ease and spread it, iOS is pretty secure and much harder to hack. That should be a HUGE deciding factor, especially if you do things like mobile banking on an Android phone. Just asking for trouble.
My deciding factors were simple. I was tired of buying new Android phones that were always 1 to 2 updates behind, even at launch. Sick of carrier bloatware and my S3 would do odd things. Lag while typing, screen would go black, although I still had sound, so I had to pull the battery about once a week. No integration with my Apple products or car stereo etc..
With the iPhone I like having an up to date OS as it should be, integration with my Apple devices and car stereo, better build quality (especially earpiece and speaker), better quality and selection of apps, more intuitive voice commands with Siri, better speech recognition, better battery life, better photo/video quality, better display, better GPS and signal. Thats just off the top of my head