The question cannot be simplified in a singled out formulate, I think the answer should be based on two factors of consideration.
This question can be shorten as, why is it the case that, Chinese culture could easily ate up non-Chinese ethnicity in the past?
Before the answer itself, I HAVE TO make a clarity on the ontological definition of “Chinese”, now adays we thinks of it as an ethnic group, a nation in the same way like English , French, Viet Nam kihn and Japanese , but this is a disputable view within the Vietnamese Sinophile, some Vietnamese scholars considers that, an ethnic group is rooted in a myth of sibling-hood among people who are believed to be having a common ancestor in the family blood line sense, based on this theory, some Vietnamese scholars considers that, Chinese isn’t an ethnic group, because it isn’t believed to be a bloodline relatives group, although bloodline could also involves in some people, but rather, it’s requirement of being a Chinese, it is rooted in pledging allegiance to a civil system of “central kingship ruling”, hence, “Chinese” is an umbrella term for a variety of ethnic groups which engages in the belief on a civilized central kingdom; Just that from 19 century on, China had been heavily kicked, and it cannot continue to dreams of itself being the central kingdom, especially the entire globalization heavily hits the head, therefore, the regime of R.O.C’s initiated a step to refine “Chinese” as an ethnic group, just as what Jews do to their identity while seeing the decline of a systematic Judaism in unified way, after R.O.C fell it’s ruling in mainland, P.R.C continues such legend , but yet, they had to follow the footstep of USSR’s propaganda on having diverse ethnic groups being unified for international Communist movement, P.R China then arbitrary divides some ethnic minority, but we can notices that, the members of these minority aren’t aware that they are ethnic minority until they check their residential book lol, how did P.R.C worked on this arbitrary division among people? I don’t know, but there are plenty of options to do so, if you give a strategy online game in simulation to that work, I can divides more than 56 groups.
Whether or not Chinese is an ethnic group or a civil systemic nation likes the case of America nation in the United State nowadays,this is a disputable issue among Vietnamese scholars on this matter. I personally agree with the view that, Chinese is not a unified ethnic group, and it had been leaded by the early generation of people to preached this civil system to outsider, usually via war of course.
Now, begin to answer your question:
1st factor, the long years of China’s invasion toward others within the Far East.
China had a long years of invasion toward others within the Far East, China had been a very barbaric empire ever since it’s first systematic dynasty was formed, it’s initiative of conquering was rooted in their idea of building “unified civil system right below heaven”, as their very conception of considering itself being the central kingdom right below heaven, this is a notion instead of being necessary taken as a doctrinal belief, a Chinese is someone who is either consciously or unconsciously holding this notion about his or her own identity, that being the central kingship person in comparison to the non-Chinese people, they have a duty to civilizes the others, to rules them under their guardianship. Then let’s look at their invasion throughout the history, it was very savage, and it enjoyed a successful invasion to the entire Viet land (south below Yangtze river), in comparison to the other regions which were always targeted by China, such as, Mongolia, Turkistan, Tibet and Manchu, China’s invasion to Viet is in a successful case, Vietnam surely wouldn’t become an exception of sinicization.
Worthy to be kept in mind is though, Viet Nam being the first nation-state in Asia, her unique identity was emerged via a process of SPENDING EFFORT TO COMPLETES de-sinicisation, but yet, she is by result very much sinicised, so, why is that? The reason why, Kihn people were able to keep the Chinese army stayed out from being unified into the direct ruling of China, because some compromises were taken proactively, such as to adopt the Chinese system, as done for responding the Chinese superiority, here is an example to illustrates the motivation of it, during the Indochine era, French proclaimed that, Viet Nam is a “little dragon”, she has no uniqueness on her own, and hence, let French educates her about how to be civilized in a non-Chinese way, so that she can get rid of the Chinese influences, therefore, Viet Nam shall not try to kick the french out, to response such French propaganda, Vietnamese intellectuals would have to presented that, Viet Nam doesn’t need a full guides from France, she is able to forms her civilization, their presentation was mostly mixed with European ideas, to demonstrates that, Viet Nam already can manifests those ideas on her own, similar way that previous rulers of Viet Nam adopt Chinese civilized system to response the Chinese.
2st, the result of Chinese culture being useful for Manchurian ruling over Chinese people, with Manchurian ever being minority during Qing Dynasty.
Chinese civilization in it’s result, it has been very much dominated in the far east, and it is difficult for any ruler to rules the Chinese without exercising Chinese cultural belief, so, we have Yuan which is the major portion of Mongolian Empire, for the sake of ruling the Chinese, they also ran a quasi Confucius system, as majority of citizens at that time came from the Chinese Dynasties, I bet that the Yuan governors faced a struggle which nowadays many of us also partakes, which is, it is very hard to communicates with the Chinese way of thinking, language isn’t even gonna to help, and Chinese people conceals their way of thinking, make it radically isolated from us, unlike for other cultures, we are able to theorizes them, then to builds a mutual platform of understanding, but not the case for Chinese culture. Therefore, when Manchurian established the Qing State to ruled China, I can assure that, they faced the same issue, question of “how to rules these people?”
As Manchurian were actually a minority group during Qing era,they didn’t invaded China with their mass migration, as the Qing regime continued the Chinese ruling, education on Manchuria culture eventually became marginalized, people in general including Manchurian, they simply couldn’t express a interested on Manchurian culture, having examination system which is in Chinese, normal Manchu would rather adopt Chinese education, same for the Emperor himself, in order for him to convinces his fellow Chinese citizen that, he is a rightful ruler assigned by the heaven. As time passed, now Manchurian no longer exist in reality, but a useful label for P.R.C and R.O.C to tags(or you may says to respects in some extend) on some Chinese who can traces their ancestry to Manchurian tribes.
In short, the answer to this question is, it is simply a historical result, not because of the power of Chinese culture, and we can considers a lot of historic alternative which can ruined Chinese out of cultural map, just that it didn’t happened, due to the two factors that I mentioned above, 1st, the long years of China’s invasion toward others within the Far East, 2st, the result of Chinese culture being useful for Manchurian ruling over Chinese people, with Manchurian ever being minority within the land of China.