I've seen the point made Rome is an outlier in that we know so much about it compared to other peoples at the time. I see this chalked up to the Romans writing a lot down, but what about Persia and China?
They were equally massive and bureaucratic; I imagine they had similar educated elites. So why don't we have the same level of detailed information on them?
Short answer:
There is less information about Persia, but in the case of China you are mistaken-- there is far more historical material than you are aware of, probably significantly more than there is for Rome.
Discussion
In the case of Persia, its that we lack any of their own historical accounts. We've got inscriptions, we've historical accounts by others -- most famously Greeks, works like Xenophon, Herodotus and others surviving only in citations by other ancient historians, like Ctesias' Persica. But, sadly, we don't have any Persian historians (by "historian" -- I mean someone like Herodotus or Livvy, eg a man writing of his own time, or near it). Whether such histories once existed and were destroyed (perhaps by the Sassanians, perhaps by the Arabs?) is unknown.
In the case of China, you've been misinformed. Walter Swingle characterized Chinese history as "the paradise of the historian" for the volume of sources. . . and Swingle was writing a century ago; we have far more than he knew of, and it's a reasonable assumption that future generations of historians of China will have more to read than we do.
The volume of Chinese historical material is massive. In terms of ancient historians, the one who leaves us the most is Sima Qian, completing a work begun by his father Sima Tan: the "Shiji" (= "Records of the Grand Historian") . This is a massive work, from the first century BCE; later dynasties took it as an example and there are many subsequent historians, and much of it survives.
Add to that the voluminous materials produced by the Chinese bureaucracy and a contemporary historian of, say, the Tang dynasty, has mountains of contemporary or near contemporary material to read, probably more than there is for a Greek or Roman historian. And of course there's a huge number of inscriptions on monuments, graves, etc
So _you_ likely know more Roman history than Chinese history . . . but that's just what you've been reading. Westerners rarely read much Chinese history, that doesn't mean it's not there. As anyone who's picked up Joseph Needham's "The History of Science and Civilization in China" some 20 -odd books (Needham divided it into 7 "volumes", but for example "volume 5 on Chemistry and Chemical technology" is actually 13 books) knows "there is far more Chinese history than I knew, know or could possibly know if I read from now to the end of my life"
Sources:
Walter T. Swingle. “Chinese Historical Sources.” The American Historical Review, vol. 26, no. 4, 1921, pp. 717–725.