What would be the reason for chosing Go over of one of the existing mainstream languages? Even if it's "better" (cleaner, more expressible, more readable etc.)

What would be the reason for chosing Go over of one of the existing mainstream languages? Even if it's "better" (cleaner, more expressible, more readable etc.) than say C++ or Java, I don't see that outweighting maturity, availability of libraries/SDKs/tools, availabilitiy of developers for one of the current mainstream language.