2 main reasons, Usability and Cost:

Usability is one of the main reasons why the iPhone sold almost 30 million units (as of Q2 2009) in a little over 2 years. Most people regard the iPhone’s usability as ‘experience changing’. It raises the bar of how touch screen devices should be built. The high usability makes the iPhone and the iPod Touch the best input device for general data collection to-date.

The second reason is the cost of the device. A traditional data collection device (like the ones from Symbol Technologies) cost over $1000.00 a piece. With the iPhone and the iPod Touch, one can start with as little as $99.00 for an iPhone 3G or $229.00 for an iPod Touch.