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Because my iPhone 4 is now out of warranty and I hate carrying around an expensive device without a warranty. Also, because I was eligible for a full upgrade. Lastly, I wanted to move up to a 32gb model since I was running out of space on my iPhone 4.
Other than that, the camera was a huge consideration, since I no longer take my point-and-shoot with me anywhere.
Siri and the speed bump are just nice bonuses.