Google had a private dogfood beta in 2014; I remember being intrigued by it but I wasn't eligible because I'd just moved farther away and my commute now included highway. (It was for surface streets within 5 miles of the Googleplex only.)

Not sure why it's taken so long since...my guess is that they're being super cautious on the regulatory front and also might not have an idea on go-to market strategy.