My handwaving analysis would be that advertising around search is slowing and instead mostly growing around content - this is because the web has matured and users are now educated on what it has to offer. They spend less time searching and browsing the web and instead spend most of the time consuming content (via streaming services and social feeds) through established platforms (youtube, instagram, netflix, spotify, twitch, etc.). And Alphabet's main content platform youtube is somewhat harder to monetize.