We are back to this discussion from over a year ago for the who knows what time again?
There's a very simple answer (and thousands more discussed in probably dozens of other existing threads on the topic from the past year or so): after 6 versions of the OS looking mostly the same Apple figured it was time to change things as that's how things work in life when it comes to pretty much any product out there, and especially one as dynamic and evolving as a mobile OS these days.
Consumers often equate change/innovation by how the layout and graphics of the UI look. The change in IOS 7 was to address that very issue. It is unrealistic to expected Apple to change the UI only one year after doing so.