Having briefly interacted with the founders of Google a couple of times, I'm pretty sure it's by design. I think that at some point, they recognized that Google must turn into a big, boring company that communicates in platitudes. There is no other way to do it when you have 150,000 employees, trillions of dollars at stake, and every regulatory agency in the world wants a piece of you. Even in the late 2000s, when Google was one tenth the size and was near-universally loved, the model of having frank fireside conversations with execs was already stretched thin.
They basically decided they want to reap the benefits of the company's success, but work on pet projects and not be the face of this anymore. Sundar was and is a perfect pick for a CEO - that is, if you accept that Google really couldn't outrun fate.