Read the book What Would Google Do?!

Their business models peaked at the right times and their brand is sticky because the technologies work. They make the complicated world of information on internet simple to navigate. Like the Apple products, it is a tool that anyone can use and feel good about. As long as you listen to your customers and the results continue to be positive, it is hard for anyone to take your market share... even Yahoo!

A Series A round title of the round may also have been required because a new share structure was being created to satisfy the investors requirements given the larger amount of money they were investing. The more money, the greater the perceived risk. The new share structure may have provided a greater reward or multiple in the event of an exit or public offering, as well as created new 'preferences' relative to the common shares likely already in place from the true start up phase (or angel round).

To understand more about series funding from series seed to series E check this out: https://www.startups.com/library/expert-advice/series-funding-a-b-c-d-e