I think for a variety of reasons large platforms are moving to a more simpler UI design:
Less cpu intensive. Let's face it, especially on mobile platforms, the less resources you use on the UI, the better off you are, unless your application is UI-centric.
Easier to manipulate. This is more a reason based on theory. Take a vector image versus a static jpg for example: To scale, tween or generally alter a vector image is less prone to pixel artifacts than a jpg. This makes transitions such as movements, alpha fade-in/outs, and scaling easier on the eye.
Neutral is good. This is another reason based on design theory. When you have a framework such as an OS or an application based on creation, you want your UI to be as neutral as possible in order to remain unbiased during the creation and/or display process. Let's use iOS for example, you want the icons of apps to be as uniform as possible and neutral in order to facilitate all the variety of apps and content that will be displayed in order to maintain an aesthetically pleasing look. By simplifying the UI, layouts will remain cohesive & pleasing, unlike the 1990's Geocities websites that plagued the internet for far too long.
Also by maintaining a simplistic neutral UI, the attention can be maintained on the content or app, rather than the UI. Skyrim is a great example of a simple elegant UI that does not detract from the action of the game.
Corporate Agility. This is probably the most important reason for companies such as Windows and Apple. The simplistic UI is easy to translate unto multiple platforms. You can rest assured that the metro tile look will display just as good on a phone as it will on your xbox, while retaining the familiar tile look you are becoming accustomed too, which is the brand they hope catches on.
