With 67 percent search market share, when Google speaks, search marketers listen. Google states that responsive web design is its recommended mobile configuration, and even goes so far as to refer to responsive web design as the industry best practice.
This is because responsive design sites have one URL and the same HTML, regardless of device, which makes it easier and more efficient for Google to crawl, index, and organize content. Contrast this with a separate mobile site which has a different URL and different HTML than its desktop counterpart, requiring Google to crawl and index multiple versions of the same site.
Additionally, Google prefers responsive web design because content that lives on one website and one URL is much easier for users to share, interact with, and link to than content that lives on a separate mobile site. Take for example a mobile user who shares content from a mobile site with a friend on Facebook who then accesses that content using a desktop, which results in that user viewing a stripped down mobile site on their desktop. This creates a less than optimal user-experience, and because of the large emphasis Google is now placing on user-experience as a ranking factor, this is essential to take into account with regards to SEO.