Some say the change is driven by high-DPI displays. I disagree. I don't see any intrinsic reason that flat designs look better than rich, three-dimensional designs on a high-DPI display. Without a doubt, flat can look nice, but so can things like this:
http://www.sequelpro.com/blog/wp-content/uploads/2013/01/seq...
Another justification I've heard is that it's a reaction to the excesses of the previous trend. People often point to the leather motif in certain Apple applications as an example of such excess. But first of all, those examples are outliers; few designs actually went that far. Second, the existence of a questionable use of a given style is not an effective argument against that style in general. Third, "some things were extremely 3d, so now we'll be extremely flat" seems like contrarianism for contrarianism's sake.
[1] Some call this skeuomorphism. I tend not to, because the term technically means something narrower than what we're talking about: http://en.wikipedia.org/wiki/Skeuomorph