Do you remember the last time you went to a mobile site and had an — ahem — unmovingexperience? (Pun intended.) Maybe the site wasn’t responsive. Or perhaps it was really difficult to find what you were looking for. Or, maybe it just loaded really slowly.
Whatever it was, you may have left to go to another site as a result. Google knows that unhappy website visitors will go elsewhere, thereby increasing bounce rates and decreasing the chances a site will rank on mobile searches…