I personally think they did this because Sergey Brin and Larry Page were getting bored with day-to-day Google operations but wanted to remain at the top. They can't just shift their focus on the new toys they're building like Robotics, Fiber, Vehicles, etc. So best bet, name Sundar Pichai as the new Google CEO, put Google under Alphabet, become the new CEO of Alphabet, and that allows them to get rid of day-to-day Google search operations and focus on their shiny new toys.