It's a trend and, like any trend, governed by nobody. In every generation you have people who want to move away from the established, who want to renovate and innovate. Old trends come back or new trends are being formed.
Flat design was a logical reaction on skeuomorphism. It's the exact opposite. In every trend somebody is the first, but that's not the person that governs it. Windows 8 made flat design popular, but they did not set the trend nor do they govern it. Flat design can easily be made obsolete if a large enough group of people would say flat design is outdated and start using skeuomorphism again or create an other trend. The same goes for money. If a large enough group of people says I don't put trust anymore in coins and bills, but from now on I trust seashells, in no time we would all be trading in seashells.
There have been a lot of talk about flat design and it's merits over skeuomorphism.
So why do icons and logos look better flat? They don't necessarily. Some people really think it is better looking. Others just think they think it's better looking because it's the trend. But like any trend, several months, a year or several years from now something else will be 'better-looking'.