YouTube has adjusted its search algorithm to emphasize videos that users watch more.
Over the past few months we have made some changes to YouTube to encourage people to spend more time watching, interacting, and sharing with the community. To support this, we’ve updated what we call video discovery features, meaning how our viewers find videos to watch via search and suggested videos. These changes better surface the videos that viewers actually watch, over those that they click on and then abandon. — YouTube Creator Blog