It sometimes seems like China is the chinese equivalent of the Roman Empire, but we in Europe ended up with different languages, while it seems that China completly unified in a single han-chinese culture.
Compare China to India, which looks a lot more diverse and probably wouldn't even be a country without the involvement of the British Empire.
Also China has a tendency to be ruled over by a small noble elite, often originating from outside of china like the mongols for example.
I guess this is also an inadvertant question in regards to why there was no revival of the Roman Empire or why the latin assimilation wasn't as succesful. Why was there no Alexander the Great type personality who took over much of Europe or if those types existed, why did they fail?
/u/Voidjumper_ZA covers a couple points - there's also the assimilation of various non-Han ethnic groups through their adoption of Chinese culture, which is another major point of its supposed homogeneity. However, it's important to realise that this idea of cultural 'one-ness' doesn't apply to many ethnic Chinese themselves, as this is more of a eurocentric/Western construct of culture - Chinese people recognise differences not only between ethnic groups but also between locations, and from this spring a whole host of different dialects, customs, and so on, that aren't visible to the outside eye. We 'see' China as a homogeneous country because our exposure to their custom is mostly the stereotypical 'Orient' rather than any deep insight into the ethnic differences. Also - other than the Yuan and Qing dynasties, most of China's historical dynasties has seen it being ruled by ethnic Chinese people.
Well for one, the British Raj in India only lasted from 1858 to 1947, which is a total of 89 years. Compare that to the numerous dynasties that have stretched over China for thousands of years. Here's an image which demonstrates what I'm talking about. This had a much greater affect at unifying the culture and country as the length of dynasties stretched over sets of generations and as power and fame moved, so to did court officials and the gentry, taking up and modelling themselves after whomever was in power at the time to win favour.
Another point is the sheer scope of the dynastic influence over the country we now know as China. As the dynasty's sizes and controls gained, more and more areas would fall in line with the predominant culture of the time. Here's a gif to illustrate the territorial expansion of the dynasties: link.
Contrast this to India where much smaller regions were ruled over by regional kings, and those regions kept a lot more of their individual culture and identity. And when Empires did spread across the subcontinent they seemed to fall relatively quickly and then remain fractured for extended periods of time before another empire would take over again.
However China is by no means a homogenous country. The eastern coastal regions, sure maybe, but the rest is hardly one singular group or language. We often refer to the language spoken there as "Chinese" however this is not correct. Mandarin is the proper name of the most widely spoken variety of Chinese. However as you can see here (warning: Wikipedia) and here (non-Wikipedia) Chinese comes in many varieties. (You're also welcome to have a look at this site however it's a little bit messy.)
And the same works with ethnic groups. If you want a quick glance over of how many exist you can take a peak at (again warning: Wikipedia) this page to get an overview of how many co-exist with each other. You can also use the UN Data page for seeing census data for different ethnic groups in China, here, and then make sure to filter by "China." Many of these groups, however, do just end up referring to themselves as "Han" as the term carries quite a lot of prestige in modern China.
Here's also a short little video which can give you a nice look into how the Chinese don't even see themselves as a single "Chinese" state: link.
Mods: I don't know if I've provided enough sources. If you want more please let me know for which items you'd like it and I'll try my best to provide it.
