Here's a maybe or two.You'd think that after three years of the same design they would have done adequate planning to avoid delays.
Maybe Apple planned to bring this tech out last September but due to the technology not advancing at a fastest enough rate Apple had to use the 6/6s design form again for the 7.
Maybe it wasn't the plan but the tech not being at the point Apple thought it would be at this point forced them into the same for factor.
Maybe, it was/is a little like what happened to the Mac Pro. "Things" didn't go the way Apple anticipated and so Apple had to make adjustments to their internal plans.
Maybe, that is why Apple doesn't put out press releases on what products it will be coming out with.