The irony. I would disagree; it picked up because people tried to make the case that it was more user-friendly under the guise that simplicity/cleanliness = understandability at the expense of affordance. Some interfaces can get away with this, others cannot. I'd say that most cannot, simply because most designers are designing out of aesthetics, not fundamental usability.
Commenter Trevor has it right:
> It's amazing the justifications being used for a new design trend.
> No, a flat design is not required because of mobile. In fact, with the advancements of CSS and HTML, which mobile phones are driving, designers can do non-flat designs easier than they ever could. If there ever was a time when flat design was a required because of technological constraints, it was 15 years ago when all we had was CSS1.
> Nor is flat design "less distracting". A musical note tells no more inherent of a story than a shiny sparkling CD. Icons exist within the cultural context. Now that CDs are becoming obsolete, a musical note might make more sense to indicate music. But whether that icon is rendered flat or not has no bearing on it's usability.
> The flat design trend is just that, a trend. Designers get bored of using the same style. So a new style emerges. Technologies and culture may provide minor tweaks and constraints, but most of it is driven by the fact that the current design has been around for a couple years now and designers want to try something new.
> Let's accept the new trend as a trend and stop trying to justify why it is "better" in some absolute sense.