Also, it's next in their very predictable naming scheme.
People focus way too much on what it's called. I'd be very surprised if they don't eventually get away from the 5, 5s, 6, 6s, etc. scheme. Most of their other products have gotten away from it, including the iPad. They've already started with the iPhone SE.
They'll probably call this one the iPhone 7 because it's rumored to keep with similar styling, but with the (rumored) complete refresh next year, they'll probably just call it the iPhone.
To be fair, the SE is widely regarded as an "out-of-band" update - a hardware rev of the iPhone 5S. I'm not sure how likely it is that its name is predictive of a new naming scheme.
People focus way too much on what it's called. I'd be very surprised if they don't eventually get away from the 5, 5s, 6, 6s, etc. scheme. Most of their other products have gotten away from it, including the iPad. They've already started with the iPhone SE.