Simple,Since the PRC is in Power they have dictated that Mandarin(汉语，国语，普通话） is the national language.Politics, thus the current business language is mandarin,and seeing as every school in the PRC teaches in Mandarin meaning that everyone can atleast comunicate in mandarin, this would be a good place to start.This is unlikely to change.
My advice is learn Mandarin first,then go learn your dialect but learn Fantizi繁体子 (traditonal characters) first or as well as jiantizi简体子 (simplified) which will aid you in many areas of Asia.