Brin and Page are natural cofounders. I guess the maturity of Google is boring to them. They want to start projects, not manage them once they’re established. This a good excuse to create Alphabet and leave Google in the best hands: Sundar Pichai seems really competent.
I don’t really get why Google is part of Alphabet when this company is much important than the rest combined. At least, it is now. Anyway, it gets distractions apart from Google, that now can focus on its core business.
Update: The Verge has a good explanation of the new structure. Now everything makes sense.