The whole iOS 7 / flat UI thing that blew up at WWDC 2013 still hasn't, for me anyhow, come to a simple conclusion as to why the flatness. Board after board of kids going nuts at each other but not really focussing on why.
As screens get thinner, think TV and Google Glass, and interfaces on any surface ubiquitous, it seems to me that having a thick button on a thin interface isn't going to fly.
Gruber, as usual, knocks it out of the park:
The primary problem Apple faced with the iPhone in 2007 was building familiarity with a new way of using computers. That problem has now been solved. It is time to solve new problems.
It's a flat, flat world.