I think it's change in leadership.

Sergei and Larry seemed to enjoy supporting wild ideas and were happy to put money up for ideas that would very likely never be profitable. They had their run with these bets, none of them particularly worked out and they stepped away (as running the ad business is far less fun than coming up with a breakthrough search algorithm when they started).

Sundar and Ruth are just closer to what you would expect from a normal publicly traded company leadership (a lot like other 2nd+ generation big-tech CEOs). Focus on revenue, profit and cutting loss-making bets. Clearly very good at focusing on profitability, but at the expense of crazy bets, which I very much used to appreciate about Google