A bigger iPhone was a no brainier 2 years ago at that time andriod began to surpass Apple in sales/ Probably largely because people believed they were one-upping the iPhone. While I'm sure most of those people have found that to not actually be the case, a larger iPhone would be one-upping the iPhone. Then there would be very little reason to buy a galaxy. It's frustrating to me that they've waited so long. My wild guess is that the product pipeline for the iPhone 5 and 5S was Steve's last instruction. Tim chose not to deviate, partly out of respect, and partly because he didn't want to rock the boat as Apple's new leader.
Also, I would love it if Apple got rid of iPhone generation numbering, and went with a series. The iPhone A, B, & C series. Like luxury car manufacturers do.