Benefits of a smartphone: people always have them on hand.
Benefits of an ereader with a reflective display: sunlight readability and battery life.
Benefits of a tablet: easier to use for fixed format documents.
The type of reading that people do and their reading habits will dictate the type of device that they read on, because each type of devices has its benefits and drawbacks. Personally I use a combination of a tablet and an ereader with an eInk display. One is for large format documents, the other is for fiction (and certain non-fiction). When I lived in a place that had cell service, I used the phone for reading too. Of course, most people will prefer a single device to do their reading so they will choose one or the other. I'm not in the least bit surprised that cell phones would top the list because of the convenience factor.