Reading can answer that better than I; however, making the skin itself ​is only part of the issue – you also would want to scrap m.wikimedia.org etc. ​Right now, the mobile sites have to make massive changes to the content to make it fit on​ a mobile screen (and even then can't fix some things, like tables). Giving mobile users a responsive skin whilst the contents weren't appropriate wouldn't make anyone happy. Until the contents of at least most the millions of pages of Wikimedia wikis' projects are mobile-safe, we can't reasonably get rid of the mobile "site".

J.