Some people wanted an all around bigger screen. Others wanted to keep the same small body. I don't know why either. I remember when the iPhone 4 leaked people called it fake, unlike Apple, and hated it.
16:9 as the change is a bit daft, though, unless the interface has a big overhaul. A shitty part of the iphone is typing in text boxes, like the ones I'm typing in now. Keyboard takes up most of the screen in landscape leaving about a line to see what you're typing. Similarly, typing in portrait mode will have the same cramped keyboard.
