China has 56 ethnic groups, I always wondered how some of them resisted assimilation to the dominate "Chinese" culture.
Forgive the quick answer to this question. I can expand more in a few hours if you're interested or if there are specific sub-points you want more information on.
Note that this number you're quoting is actually not a reflection of any objective reality; China does not have 56 ethnic groups. It simply had 56 official classifications. Many of these include a great many other groups, such as the Gaoshan, which includes over a dozen distinct Austronesian ethnicities/languages/cultures, all combined into one by the government of the PRC.
A big reason you have Zhuang and Hmong (Miao) surviving as they have today is the very fact that they are counted among the 56. By being counted among these minority ethnicities (often called Nationalities), there are certain language rights afforded to the Zhuang and Hmong. Minority language education is permitted to occur in the areas which are officially recognised as belonging to one of the minority groups. These minority language rights are somewhat new, having been put into place since the Reform & Opening period starting at the end of the 1970s.
Still, as recently as this past year there has been extra attention paid to the lack of Mandarin proficiency among many of these communities. But for what it's worth, none have entirely been free of influence of Han culture. Even since the Reform period, the actual implementation of policies has not really been in line with what's written in the laws. But then we're getting close to a 20-year rule violation.
Sources (for further reading if you're interested):